Assertion : Carbonic anhydrase is present in the erythrocytes.
Reason : In erythrocytes the carbon dioxide combine with water and is transported.

Options

A. Statement A is correct and is responsible for statement B
B. Statement A is not correct, but statement B is correct.
C. Both statement A and B are wrong .
D. Statement A is correct, but not involved in statement

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Carbonic anhydrase is present in the erythrocytes."
- **Explanation**: Carbonic anhydrase is indeed an enzyme found abundantly in red blood cells (erythrocytes). Its primary role is to facilitate the conversion of carbon dioxide and water into carbonic acid.

2. **Understanding the Reason**: The reason states that "In erythrocytes, the carbon dioxide combines with water and is transported."
- **Explanation**: In the erythrocytes, carbon dioxide (CO2) diffuses into the red blood cells from the tissues. Here, it combines with water (H2O) in the presence of carbonic anhydrase to form carbonic acid (H2CO3). This reaction is crucial for the transport of carbon dioxide in the blood.

3. **Linking Assertion and Reason**: The presence of carbonic anhydrase in erythrocytes is essential for the reaction described in the reason.
- **Explanation**: The enzyme catalyzes the reaction that allows carbon dioxide to combine with water, forming carbonic acid, which is then dissociated into bicarbonate ions (HCO3-) and hydrogen ions (H+). This process facilitates the transport of carbon dioxide in the bloodstream.

4. **Conclusion**: Both the assertion and the reason are correct, and the assertion supports the reason.
- **Final Statement**: Therefore, the answer is that both statements are true, and the assertion (A) is the reason for the reason (B).