Residual air mostly occurs in 

Options

A. alveloi
B. brounchus
C. norstrils
D. trachea

Answer

**Step-by-Step Solution:**

1. **Understanding Residual Air**:
- Residual air refers to the volume of air that remains in the lungs after a forced exhalation. It is crucial because it prevents the lungs from collapsing and ensures that there is always some air available for gas exchange.

2. **Identifying the Location of Residual Air**:
- The question asks where residual air mostly occurs. To answer this, we need to consider the structures involved in the respiratory system.

3. **Analyzing the Options**:
- The options provided are:
- Alveoli
- Bronchus
- Nostrils
- Trachea

4. **Evaluating Each Option**:
- **Alveoli**: These are tiny air sacs in the lungs where gas exchange occurs. They contain residual air.
- **Bronchus**: These are the main passageways that direct air into the lungs but do not primarily hold residual air.
- **Nostrils**: These are the external openings for air intake and do not contain residual air.
- **Trachea**: This is the windpipe that connects the throat to the lungs but does not store residual air.

5. **Conclusion**:
- Based on the analysis, the correct answer is **Alveoli**, as this is where residual air is primarily found.

**Final Answer**: Residual air mostly occurs in **Alveoli**.

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