Assertion: Food chain is responsible for the entry of harmful chemicals in our bodies.
Reason: The length and complexity of food chains vary greatly.

Options

A. Both A and R are true, and R is correct explanation of the assertion.
B. Both A and R are true, but R is not the correct explanation of the assertion.
C. A is true, but R is false.
D. A is false, but R is true.

Explanation

To solve the question, we will analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Food chain is responsible for the entry of harmful chemicals in our bodies."
- This is true because harmful chemicals, such as pollutants, can accumulate in organisms at different trophic levels through a process known as biomagnification. As one organism consumes another, the toxins can become more concentrated in the bodies of predators.

2. **Understanding the Reason**:
- The reason states that "The length and complexity of food chains vary greatly."
- This statement is also true. Food chains can vary in length (number of trophic levels) and complexity (interconnected food webs) depending on the ecosystem.

3. **Evaluating the Relationship**:
- While both the assertion and the reason are true, we need to determine if the reason correctly explains the assertion.
- The assertion discusses the entry of harmful chemicals into our bodies through the food chain, while the reason discusses the variability in length and complexity of food chains.
- The variability in food chains does not directly explain how harmful chemicals enter our bodies. Therefore, the reason does not serve as a correct explanation for the assertion.

4. **Conclusion**:
- Both the assertion and the reason are true statements, but the reason does not provide a correct explanation for the assertion. Thus, the correct choice is that both are true, but the reason is not the correct explanation for the assertion.

### Final Answer:
- Both assertion and reason are true, but the reason is not the correct explanation for the assertion.

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