Mammals are different from birds in all these characteristics except :

Options

A. hair
B. mammary glands
C. constant body temperature
D. young born alive

Explanation

To solve the question "Mammals are different from birds in all these characteristics except:", we need to analyze the characteristics provided and identify which one is common to both mammals and birds.

### Step-by-Step Solution:

1. **Identify the Characteristics**: The characteristics provided are:
- Presence of hairs
- Mammary glands
- Constant body temperature
- Young ones are alive

2. **Analyze Each Characteristic**:
- **Presence of Hairs**: Mammals have hairs on their bodies, while birds have feathers. This characteristic is different between the two groups.
- **Mammary Glands**: Mammals possess mammary glands that produce milk to nourish their young, whereas birds do not have mammary glands. This characteristic is also different.
- **Constant Body Temperature**: Both mammals and birds are homeothermic, meaning they can maintain a constant body temperature regardless of the external environment. This characteristic is similar for both groups.
- **Young Ones are Alive**: Mammals are viviparous, meaning they give birth to live young, while birds are oviparous, laying eggs. This characteristic is different.

3. **Conclusion**: The only characteristic that is not different between mammals and birds is the ability to maintain a constant body temperature. Therefore, the answer to the question is "constant body temperature".

### Final Answer:
Mammals are different from birds in all these characteristics except: **constant body temperature**.