Visiting high mountains may cause altitude sickness in men living in plan areas. Prime of this is

Options

A. excess of `CO_(2)` in blood
B. decreased efficiency of haemoglobin
C. decreased partial pressure of oxygen
D. decreased efficiency of red blood cells.

Answer

### Step-by-Step Solution:

1. **Understanding Altitude Sickness**: Altitude sickness, also known as hypoxia, occurs when individuals ascend to high altitudes where the oxygen levels are significantly lower than at sea level or in plain areas.

2. **Oxygen Levels at Different Altitudes**: At high altitudes, the atmospheric pressure is lower, which results in a decrease in the partial pressure of oxygen. This means that even though the percentage of oxygen in the air remains the same, the amount of oxygen available for breathing is reduced.

3. **Breathing Rate Changes**: When a person from a low altitude (plain areas) travels to a high altitude, their body responds to the lower oxygen availability by increasing the breathing rate. This is an attempt to take in more oxygen to compensate for the reduced partial pressure.

4. **Impact of Partial Pressure**: The breathing rate is closely linked to the partial pressure of oxygen. In lower altitudes, the partial pressure is higher, allowing for a normal breathing rate. In contrast, at high altitudes, the lower partial pressure leads to a faster breathing rate as the body tries to obtain sufficient oxygen.

5. **Conclusion**: The prime cause of altitude sickness in individuals from plain areas visiting high mountains is the decreased partial pressure of oxygen. This reduction in available oxygen leads to symptoms of altitude sickness as the body struggles to adapt to the new environment.

### Final Answer:
The prime cause of altitude sickness in men living in plain areas when visiting high mountains is the decreased partial pressure of oxygen.

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