A The blood vessels called arteries have thick walls.
R Blood transfusion should not be done without testing the blood group and Rh factor.

Options

A. If both As and R are correct and R is correct explanation of As
B. If both As and R are correct but R is not correct explanation of As
C. If As is correct, R is wrong
D. If both As and R are wrong.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "The blood vessels called arteries have thick walls."
- To evaluate this, we need to know the structure and function of arteries. Arteries are blood vessels that carry oxygenated blood away from the heart to various parts of the body. They have thick, muscular walls to withstand the high pressure of blood being pumped from the heart.

2. **Evaluating the Truth of the Assertion**:
- Since arteries do indeed have thick walls due to the high pressure of the blood they transport, we conclude that the assertion is **true**.

3. **Understanding the Reason**:
- The reason states that "Blood transfusion should not be done without testing the blood group and Rh factor."
- This statement emphasizes the importance of matching blood types before transfusion to prevent adverse reactions.

4. **Evaluating the Truth of the Reason**:
- The reason is also **true**. It is crucial to test for blood type and Rh factor to avoid life-threatening complications that can arise from incompatible blood transfusions.

5. **Assessing the Relationship Between Assertion and Reason**:
- Although both the assertion and reason are true, the reason does not explain why arteries have thick walls. The thick walls of arteries are related to their function of carrying blood under high pressure, while the reason pertains to the safety of blood transfusions.

6. **Conclusion**:
- Therefore, the correct answer is that both the assertion and reason are true, but the reason is not the correct explanation for the assertion. This leads us to the conclusion that the answer is option B.