Air-sacs of birds are helpful in :

Options

A. blood circulation
B. temperature control
C. reducing body weight
D. keeping body warm

Explanation

### Step-by-Step Solution:

1. **Understanding the Role of Air Sacs**:
- Air sacs are part of the respiratory system in birds, which is crucial for efficient gas exchange.

2. **Comparison with Mammals**:
- Unlike mammals, birds have a unique respiratory system that includes air sacs in addition to lungs. This adaptation is essential for their high energy demands during flight.

3. **Efficient Oxygen Supply**:
- During flight, birds require a significant amount of oxygen due to the high energy expenditure. The air sacs help in providing a continuous supply of oxygen.

4. **Unidirectional Airflow**:
- The structure of the air sacs allows for a continuous unidirectional flow of air through the lungs, enhancing the efficiency of gas exchange.

5. **Temperature Regulation**:
- The air sacs also play a role in regulating the bird's body temperature during flight, preventing overheating due to strenuous activity.

6. **Conclusion**:
- In summary, the air sacs in birds are primarily helpful in obtaining oxygen efficiently and regulating body temperature during flight.