Which of the following features of birds help them to fly?

Options

A. Streamlined body shape
B. Hollow bones
C. Strong breast muscles
D. All of these

Explanation

### Step-by-Step Solution:

1. **Streamline Body Shape**: Birds have a streamlined body shape, which means their bodies are narrow at the front and broader in the middle. This design helps them cut through the air more efficiently, reducing air resistance and allowing them to fly further.

2. **Hollow Bones**: Birds possess hollow bones, also known as pneumatic bones. These bones are lighter than solid bones, which helps reduce the overall weight of the bird. A lighter body makes it easier for birds to take off and stay in the air.

3. **Strong Breast Muscles**: Birds have strong breast muscles that are essential for wing movement. These muscles allow birds to flap their wings powerfully, enabling them to gain lift and fly effectively.

4. **Conclusion**: All of the features mentioned—streamlined body shape, hollow bones, and strong breast muscles—contribute to a bird's ability to fly. Therefore, all the options provided in the question are correct.