Difficulty in breathing on high mountains is due to

Options

A. Decrease in `pO_2`
B. Increase in `CO_2` concentration
C. Decrease in amount of `O_2`
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding the Environment at High Altitudes**:
- At high altitudes, such as on mountains, the atmospheric pressure is lower than at sea level. This affects the availability of oxygen.

2. **Decrease in Partial Pressure of Oxygen (PO2)**:
- The partial pressure of oxygen (PO2) decreases as altitude increases. This means that even though oxygen is present, it is less available for our bodies to use.

3. **Impact on Breathing**:
- Due to the decrease in PO2, our bodies sense that there is less oxygen available. As a result, we instinctively try to breathe more frequently to take in more oxygen.

4. **Increased Breathing Rate**:
- The body responds to the lower oxygen levels by increasing the breathing rate. This is an attempt to compensate for the reduced availability of oxygen.

5. **Energy Depletion**:
- The increased breathing rate requires more energy. Since the body is not receiving enough oxygen to meet its energy needs, this can lead to feelings of fatigue and difficulty in breathing.

6. **Conclusion**:
- Therefore, the primary reason for difficulty in breathing at high altitudes is the decrease in the amount of oxygen available (decrease in PO2), which forces the body to work harder to obtain the oxygen it needs.

### Final Answer:
The difficulty in breathing on high mountains is primarily due to the **decrease in PO2 level**.

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