Which is not a characteristic feature of mammals

Options

A. Diphyodont teeth
B. Ten pairs of cranical nerves
C. Seven cervical vertebrae
D. Left acortic arch in the circulatory system .

Answer

To determine which option is not a characteristic feature of mammals, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding the Options**: The options given are:
- A) Di-phyodontic
- B) 10 pairs of cranial nerves
- C) 7 cervical vertebrae
- D) Left optic arc in the circulatory system

2. **Analyzing Option A: Di-phyodontic**:
- Di-phyodontic refers to having two sets of teeth during a lifetime: the deciduous (milk) teeth and the permanent teeth.
- This is a characteristic feature of mammals. Therefore, this option is incorrect for our question.

3. **Analyzing Option B: 10 pairs of cranial nerves**:
- Mammals actually have 12 pairs of cranial nerves.
- The mention of 10 pairs of cranial nerves is characteristic of amphibians and fishes, not mammals.
- Thus, this option is correct as it is not a characteristic feature of mammals.

4. **Analyzing Option C: 7 cervical vertebrae**:
- Mammals typically have 7 cervical vertebrae, regardless of their size.
- This is a characteristic feature of mammals. Therefore, this option is incorrect for our question.

5. **Analyzing Option D: Left optic arc in the circulatory system**:
- The left aortic arch is indeed a characteristic feature of mammals.
- Therefore, this option is also incorrect for our question.

6. **Conclusion**:
- The only option that is not a characteristic feature of mammals is **B) 10 pairs of cranial nerves**.

### Final Answer:
**B) 10 pairs of cranial nerves**

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