The total number of teeth which comes only once in a life span ________

Options

A. 20
B. 8
C. 32
D. 12

Answer

To solve the question regarding the total number of teeth that come only once in a lifespan, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Types of Teeth**:
- Humans have two types of teeth: deciduous (milk) teeth and permanent teeth.
- Deciduous teeth are temporary and will eventually be replaced by permanent teeth.

2. **Count Deciduous Teeth**:
- The total number of deciduous teeth in humans is 20. This includes:
- 8 incisors (4 pairs)
- 4 canines (2 pairs)
- 8 molars (4 pairs)

3. **Count Permanent Teeth**:
- The total number of permanent teeth in humans is 32. This includes:
- 8 incisors (4 pairs)
- 4 canines (2 pairs)
- 8 premolars (4 pairs)
- 12 molars (6 pairs)

4. **Determine the Teeth that Come Only Once**:
- Since deciduous teeth are replaced by permanent teeth, we need to find out how many of the permanent teeth come only once in a lifetime.
- The total number of permanent teeth is 32, but 20 of these were previously the deciduous teeth.
- Therefore, the number of teeth that come only once in a lifetime is:
\[ 32 \text{ (total permanent teeth)} - 20 \text{ (deciduous teeth)} = 12 \]

5. **Conclusion**:
- The total number of teeth that come only once in a lifespan is **12**.

### Final Answer:
The total number of teeth which comes only once in a life span is **12**.

---