How many teeth appear twice during the life span of an individual?

Options

A. 16
B. 32
C. 22
D. 20

Answer

To determine how many teeth appear twice during the lifespan of an individual, we need to analyze the types of teeth and their occurrence in both children and adults.

### Step-by-Step Solution:

1. **Understand the Types of Teeth**: Humans have two sets of teeth:
- Temporary teeth (also known as milk teeth or deciduous teeth)
- Permanent teeth (adult teeth)

2. **Identify the Dental Formula**:
- The dental formula for children (milk teeth) is 2-1-0-2 (incisors-canine-premolars-molars) for each half of the jaw.
- The dental formula for adults (permanent teeth) is 2-1-2-3 for each half of the jaw.

3. **Calculate the Total Number of Teeth**:
- In children, the total number of teeth is 20 (10 in the upper jaw and 10 in the lower jaw).
- In adults, the total number of teeth is 32 (16 in the upper jaw and 16 in the lower jaw).

4. **Identify Which Teeth Appear Twice**:
- The teeth that appear twice in a human's life are the incisors, canines, and molars.
- Each of these types of teeth is replaced once during the transition from milk teeth to permanent teeth.

5. **Count the Teeth That Appear Twice**:
- Incisors: 8 (4 in the upper jaw and 4 in the lower jaw)
- Canines: 4 (2 in the upper jaw and 2 in the lower jaw)
- Molars: 8 (4 in the upper jaw and 4 in the lower jaw)
- Total: 8 (incisors) + 4 (canines) + 8 (molars) = 20 teeth

6. **Conclusion**: Therefore, the total number of teeth that appear twice during the lifespan of an individual is **20**.

### Final Answer:
20 teeth appear twice during the lifespan of an individual.