Which of the following is not a feature of all adult mammals?

Options

A. Vertebrae
B. Dense hair
C. External ear
D. Both (2) and (3)

Answer

To determine which of the following is not a feature of all adult mammals, we need to analyze the options provided:

1. **Vertebrae**:
- All mammals are vertebrates, meaning they possess a backbone or vertebral column. This is a fundamental characteristic of the class Mammalia. Therefore, vertebrae is a feature of all adult mammals.

2. **Dense hair**:
- Mammals are characterized by having hair or fur at some stage of their life cycle. While the amount and type of hair can vary, all adult mammals have some form of hair, making dense hair a feature of all adult mammals.

3. **External ear**:
- The external ear, also known as the pinna, is present in many mammals but is not a universal feature. For example, egg-laying mammals such as the echidna (spiny anteater) do not have an external ear. Therefore, this option is not a feature of all adult mammals.

4. **Both 2 and 3**:
- Since we have established that dense hair is a feature of all adult mammals, this option cannot be correct.

Based on this analysis, the correct answer is:

**C. External ear**