Assertion. Adenine cannot pair with cytosine.
Reason. Adenine and cytosine do not have a perfect match between hydrogen donor and hydrogen acceptor sites. Hence, they cannot pair.

Options

A. If both A and R are true and R is the correct explanation of A.
B. If both A and R are true but R is not the correct explanation of A.
C. If A is true but R is false.
D. If both A and R are false.

Answer

**Step-by-Step Text Solution:**

1. **Understanding the Assertion**: The assertion states that "Adenine cannot pair with cytosine." This is correct because in DNA, adenine (A) specifically pairs with thymine (T), while cytosine (C) pairs with guanine (G).

2. **Understanding the Reason**: The reason provided is that "Adenine and cytosine do not have a perfect match between hydrogen donor and hydrogen acceptor sites." This statement is also correct. For proper base pairing, the hydrogen bonding sites between the bases must align perfectly to form stable hydrogen bonds.

3. **Hydrogen Bonding**: Adenine and thymine form two hydrogen bonds, while cytosine and guanine form three hydrogen bonds. The specific arrangement of these hydrogen bonds is crucial for the stability of the DNA structure.

4. **Conclusion**: Since both the assertion and the reason are correct, and the reason accurately explains why adenine cannot pair with cytosine, we conclude that the statement is true.

5. **Final Answer**: Both the assertion and the reason are correct, and the reason supports the assertion.

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