[A] : A complete double circulation occurs in birds for the first time.
[R]: At no point is there any opportunity for oxygenated and unoxygenated blood to mix.

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

**Step-by-Step Solution:**

1. **Understanding Double Circulation**:
- Double circulation refers to the two separate circuits of blood flow in the body: one for oxygenation (pulmonary circulation) and one for delivering oxygen to the body (systemic circulation).
- In birds, this means that blood travels from the heart to the lungs to get oxygenated and then back to the heart before being pumped to the rest of the body.

2. **Structure of the Bird's Heart**:
- Birds possess a four-chambered heart, which includes two atria and two ventricles.
- This structure is crucial for maintaining separate pathways for oxygenated and deoxygenated blood.

3. **Function of the Heart**:
- The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs for oxygenation.
- The left side of the heart receives oxygenated blood from the lungs and pumps it out to the body.

4. **Prevention of Blood Mixing**:
- The four chambers of the heart (two atria and two ventricles) are separated by valves.
- These valves ensure that oxygenated and deoxygenated blood do not mix at any point in the circulation process.

5. **Conclusion**:
- Therefore, the statement [A] is true as birds exhibit complete double circulation for the first time, and [R] is also true as there is no opportunity for oxygenated and deoxygenated blood to mix due to the structure of the heart.