Assertion` :` In open circulatory system blood is pumped by heart passes through large vessels into open spaces or body cavities called sinuses.
Reason `:` This distribution of blood to different organs, is well regulated.

Options

A. If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion .
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertion
C. If Assertion is True but the Reason is False.
D. If both Assertion & Reason are false.

Answer

To solve the question regarding the assertion and reason related to the open circulatory system, we will analyze both statements step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that in an open circulatory system, blood is pumped by the heart and passes through large vessels into open spaces or body cavities called sinuses.
- This statement is **true**. In organisms with an open circulatory system, such as arthropods and some mollusks, the heart pumps hemolymph (the equivalent of blood) into large vessels that then open into sinuses, allowing the fluid to bathe the organs directly.

2. **Understanding the Reason**:
- The reason states that the distribution of blood to different organs is well regulated.
- This statement is **false**. In an open circulatory system, the flow of hemolymph is not as well regulated as in a closed circulatory system. The blood is not confined to vessels and can freely flow in the body cavity (hemocoel), leading to less precise control over blood distribution to specific organs.

3. **Evaluating the Relationship**:
- The assertion is true, but the reason is false. Therefore, the reason does not correctly explain the assertion.

4. **Conclusion**:
- Based on the evaluation, the correct answer is that the assertion is true, and the reason is false.

### Final Answer:
- The correct option is: **A is true and R is false.**