[A] : The heart of birds is more advanced than the four chambered heart of crocodiles.
[R]: in the bird s heart, there is a complete elimination of left aorta.

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 and R is false
D. If both A and R are false

Answer

To solve the question regarding the heart of birds and crocodiles, we will break down the assertion and reason step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "The heart of birds is more advanced than the four-chambered heart of crocodiles."
- To evaluate this, we need to compare the heart structures of birds and crocodiles.

2. **Analyzing the Heart of Crocodiles**:
- Crocodiles possess a four-chambered heart, which includes two atria (auricles) and two ventricles.
- While they have a four-chambered heart similar to mammals and birds, they also have two systemic arches (aortic arches) that can lead to mixing of oxygenated and deoxygenated blood.

3. **Understanding the Heart of Birds**:
- Birds also have a four-chambered heart with two atria and two ventricles.
- However, unlike crocodiles, birds have lost the left systemic arch (left aorta), which means they only have one systemic arch (the right aorta).

4. **Implications of the Heart Structure**:
- The absence of the left systemic arch in birds leads to a more efficient separation of oxygenated and deoxygenated blood, preventing any mixing.
- This structural adaptation is crucial for the high metabolic demands of birds, especially for flight.

5. **Evaluating the Reason (R)**:
- The reason states that "in the bird's heart, there is a complete elimination of the left aorta."
- This statement is true and serves as a valid explanation for why the bird's heart is considered more advanced.

6. **Conclusion**:
- Both the assertion (A) and the reason (R) are true.
- The reason (R) correctly explains the assertion (A) as the elimination of the left aorta contributes to the advanced nature of the bird's heart.

### Final Answer:
Both A and R are true, and R is the correct explanation of A.

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