Our Environment - Assertion-Reason

CBSE Class 10 Biology: Our Environment - Part 5: Assertion-Reason

Top 25 A/R Questions with Answers | Latest CBSE Pattern

1. MCQ 2. VSA 3. SA 4. LA 5. Assertion-Reason 6. Case Study 7. Concepts 8. One-Shot Revision

Welcome to Part 5 of our 8-part series on Our Environment. This post focuses on Assertion-Reason (A/R) Questions. These questions test your in-depth conceptual understanding and your ability to link two related statements.

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Instructions for Assertion-Reason Questions

For each question, two statements are given: one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer from the codes (a), (b), (c), and (d) as given below:

(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).

(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

(c) Assertion (A) is true, but Reason (R) is false.

(d) Assertion (A) is false, but Reason (R) is true.

Top 25 A/R Questions - Our Environment

Question 1: Decomposers

CBSE PYQ 2024

Assertion (A): Decomposers act as cleaning agents of the environment.

Reason (R): Decomposers break down dead and decaying organic matter into simple inorganic substances.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

Decomposers clean the environment *by* breaking down dead organic matter. The reason perfectly explains the assertion.

Question 2: Biodegradable Waste

CBSE PYQ 2023

Assertion (A): Biodegradable wastes, like vegetable peels, should be composted.

Reason (R): Composting converts biodegradable waste into nutrient-rich humus, which pollutes the soil.

(c) (A) is true, but (R) is false.

The assertion is true. The reason is false because the humus produced from composting *enriches* the soil; it does not pollute it.

Question 3: Energy Flow

CBSE PYQ 2022

Assertion (A): The flow of energy in a food chain is unidirectional.

Reason (R): Energy flows from the sun to producers, then to consumers, and is not recycled back to the sun.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

Energy flows in one direction (unidirectional) because the energy lost as heat at each trophic level cannot be reused by the previous level or the sun.

Question 4: Ozone Layer

CBSE PYQ 2024

Assertion (A): The ozone layer protects life on Earth by absorbing harmful ultraviolet (UV) radiation.

Reason (R): Ozone (O₃) is formed in the stratosphere by the action of UV radiation on oxygen (O₂).

(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).

Both statements are true. The ozone layer *does* protect us from UV rays. And it *is* formed by UV rays acting on oxygen. However, the *reason* it protects us is its ability to absorb UV, not how it is formed.

Question 5: Food Chain Length

Assertion (A): Food chains are generally limited to three or four trophic levels.

Reason (R): According to the 10% energy law, the energy available at higher trophic levels is too small to support a population.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

The massive (90%) loss of energy at each step means very little energy is left for the 4th or 5th trophic level, which correctly explains why food chains are short.

Question 6: Non-Biodegradable

CBSE PYQ 2023

Assertion (A): Plastic bags and glass bottles are non-biodegradable wastes.

Reason (R): They can be broken down by the action of microorganisms into simple, harmless substances.

(c) (A) is true, but (R) is false.

The assertion is true; plastic and glass are non-biodegradable. The reason is false; this is the definition of *biodegradable* waste. Non-biodegradable waste *cannot* be broken down by microorganisms.

Question 7: Biomagnification

Assertion (A): In a food chain, the concentration of harmful chemicals like DDT is highest at the top trophic level.

Reason (R): DDT is non-biodegradable and accumulates in the fat tissues of organisms at each trophic level.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

The process is biomagnification. The reason it happens (as stated in R) is that the chemical is not broken down or excreted, so it builds up at each level, leading to the highest concentration at the top (as stated in A).

Question 8: Trophic Levels

Assertion (A): In a food chain, a deer is a primary consumer.

Reason (R): A deer feeds on plants, which are producers.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

Primary consumers (herbivores) are organisms that feed on producers (plants). Since a deer eats plants, it is a primary consumer.

Question 9: Food Web

Assertion (A): A food web is a more stable system than a food chain.

Reason (R): A food web consists of a single, linear pathway of energy flow.

(c) (A) is true, but (R) is false.

The assertion is true; food webs are more stable. The reason is false; it describes a *food chain*. A food web has multiple interconnected pathways.

Question 10: Nutrient Flow

Assertion (A): The flow of nutrients in an ecosystem is cyclic.

Reason (R): Decomposers return the nutrients from dead organisms back to the soil, where producers can use them again.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

The flow of nutrients is cyclic *because* decomposers break down waste and dead matter, recycling the nutrients back into the ecosystem for producers to use.

Question 11: Producers

Assertion (A): Green plants are called producers.

Reason (R): Green plants can make their own food using photosynthesis.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

Organisms that produce their own food are called producers. Since green plants produce their own food via photosynthesis, they are called producers.

Question 12: CFCs

CBSE PYQ 2023

Assertion (A): The use of CFCs (Chlorofluorocarbons) is a major environmental concern.

Reason (R): CFCs cause the depletion of the ozone layer in the stratosphere.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

CFCs are a concern *because* they drift to the stratosphere, where they break down ozone molecules, leading to the depletion of the protective ozone layer.

Question 13: 10% Law Calculation

Assertion (A): If plants have 1000 J of energy, the lion in the food chain Grass -> Deer -> Lion will get 10 J of energy.

Reason (R): Only 10% of energy is transferred at each trophic level.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

According to the 10% law (Reason): Grass (1000 J) -> Deer (10% of 1000 J = 100 J) -> Lion (10% of 100 J = 10 J). The calculation in the assertion is correct and is based on the 10% law.

Question 14: Vegetarian Diet

Assertion (A): A vegetarian diet is more energy-efficient than a non-vegetarian diet.

Reason (R): When humans eat plants, they are primary consumers (T2), getting more energy than when they eat herbivores (T3).

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

Due to the 10% law, more energy is available at lower trophic levels. Eating at T2 (plants) is more efficient than eating at T3 (herbivores).

Question 15: Aquarium

CBSE PYQ 2022

Assertion (A): An aquarium is an artificial ecosystem.

Reason (R): An aquarium is a man-made, self-sustaining ecosystem.

(c) (A) is true, but (R) is false.

The assertion is true; an aquarium is artificial. The reason is false because, while it is man-made, it is *not* self-sustaining. It requires human intervention (feeding, cleaning, oxygen pump) to function.

Question 16: Plastic Cups

Assertion (A): Disposable plastic cups are a better alternative to disposable paper cups.

Reason (R): Plastic cups are non-biodegradable and cause environmental pollution.

(d) (A) is false, but (R) is true.

The assertion is false; plastic cups are *worse* than paper cups. The reason is true; plastic is non-biodegradable and causes pollution, which is why the assertion is false.

Question 17: UV Radiation

Assertion (A): UV radiation is harmful to living organisms.

Reason (R): UV radiation can cause skin cancer and cataracts in humans.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

The assertion states UV is harmful. The reason gives specific examples of that harm (skin cancer, cataracts), thus correctly explaining *why* it is considered harmful.

Question 18: Ecosystem

Assertion (A): An ecosystem consists of only the living organisms in an area.

Reason (R): An ecosystem is a functional unit of biotic and abiotic components.

(d) (A) is false, but (R) is true.

The assertion is false; an ecosystem *must* also include the non-living (abiotic) components. The reason is the correct definition of an ecosystem, which proves the assertion is false.

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Question 19: The 3 'R's

Assertion (A): "Reduce" is considered the best method in the 3 R's waste management strategy.

Reason (R): "Reduce" stops waste from being generated at the source, saving the most resources and energy.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

Reducing consumption is the most effective strategy because it prevents waste generation entirely, which is better than managing waste after it has been created (like in reuse or recycle).

Question 20: Trophic Level Removal

CBSE PYQ 2025

Assertion (A): If all producers in an ecosystem are removed, the entire ecosystem will collapse.

Reason (R): Producers are the only source of food energy for all consumers.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

Producers (plants) are at the base of the food chain. If the food source is removed, all organisms (primary, secondary, and tertiary consumers) that depend on them will starve and die.

Question 21: Kulhads

Assertion (A): Using disposable kulhads (earthen cups) is a more sustainable alternative to plastic cups.

Reason (R): Making kulhads on a large scale leads to the loss of fertile topsoil.

(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).

The assertion is true; kulhads are biodegradable, unlike plastic. The reason is also true; making them causes topsoil loss. However, the reason (a negative point) does not explain *why* kulhads are *better* (a positive point).

Question 22: Autotrophs

Assertion (A): All organisms in an ecosystem are either producers or consumers.

Reason (R): Decomposers are a third type of organism that breaks down dead organic matter.

(d) (A) is false, but (R) is true.

The assertion is false; it forgets decomposers. The reason is true, as it correctly identifies this missing third group.

Question 23: Abiotic Components

Assertion (A): Air, water, and soil are abiotic components of an ecosystem.

Reason (R): Abiotic components are the non-living physical and chemical factors of an ecosystem.

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

The reason gives the definition of abiotic components. The assertion lists examples that fit this definition.

Question 24: Producers' Energy

Assertion (A): Green plants capture 100% of the sun's energy that falls on their leaves.

Reason (R): Most of the sun's energy is reflected or lost as heat.

(d) (A) is false, but (R) is true.

The assertion is false; plants capture only about 1% of the solar energy. The reason is true; the other 99% is reflected or lost.

Question 25: Montreal Protocol

CBSE PYQ 2023

Assertion (A): The Montreal Protocol was an international treaty to ban ozone-depleting substances.

Reason (R): The Montreal Protocol was signed to control global warming by reducing CO₂ emissions.

(c) (A) is true, but (R) is false.

The assertion is true. The reason is false; the Montreal Protocol was for ozone depletion (banning CFCs). The treaty for controlling global warming (CO₂ emissions) is the Kyoto Protocol.