Assertion: sinus venosus is a thin-walled sac with little muscular tissue in primitive heart.
Reason: It receives venous blood through the hepatic veins.

Options

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

Explanation

To solve the question regarding the assertion and reason related to the sinus venosus in the primitive heart, we will analyze both statements step by step.

### Step 1: Analyze the Assertion
The assertion states that "sinus venosus is a thin-walled sac with little muscular tissue in the primitive heart."
- The sinus venosus is indeed a part of the primitive heart structure.
- It is characterized by its thin walls and minimal muscular tissue, which is true for both the primitive heart and the hearts of some amphibians.

**Hint:** Look for characteristics of the sinus venosus in different types of hearts to understand its structure.

### Step 2: Analyze the Reason
The reason states that "it receives venous blood through the hepatic veins."
- The sinus venosus does receive venous blood, and in the case of amphibians, it receives blood from the hepatic veins as well as from other veins like the cardinal veins.
- This statement is also true.

**Hint:** Consider the sources of blood entering the sinus venosus in different organisms to validate this statement.

### Step 3: Determine the Relationship
Now, we need to determine if the reason correctly explains the assertion.
- While both statements are true, the reason does not specifically explain why the assertion is true. The assertion is about the structure of the sinus venosus, while the reason discusses its function related to blood flow.
- Therefore, the reason does not provide a direct explanation for the assertion.

**Hint:** Think about whether the reason directly supports or explains the assertion made in the first statement.

### Conclusion
Both the assertion and the reason are true, but the reason does not adequately explain the assertion. Thus, the correct answer to the question is that both statements are true, but the reason is not the correct explanation for the assertion.

### Final Answer
- **Assertion:** True
- **Reason:** True
- **Conclusion:** The reason does not explain the assertion.