People living at sea level have around `5` million RBC per cubic millimetre of their blood whereas those living at an altitude of `5400` metres have around `8` million. This is because at high altitude.

Options

A. people get pollution free air to breath and more oxygen is availabe
B. atmospheric `O_(2)` level is less and hence more RBCs are needed to absorb the required amount of `O_(2)` to survive
C. there is more uv radiation which enhances RBC production
D. people eat more nutritive food there fore more rbcs are formed

Answer

### Step-by-Step Solution:

1. **Understanding the Context**:
- People living at sea level have approximately 5 million red blood cells (RBCs) per cubic millimeter of blood.
- People living at high altitudes (5400 meters) have around 8 million RBCs per cubic millimeter.

2. **Identifying the Key Factor**:
- The main difference between these two groups is the altitude at which they live.

3. **Analyzing the Conditions at High Altitude**:
- At high altitudes, the atmospheric pressure is lower, which results in a thinner air composition.
- This thinner air means there is a reduced availability of oxygen (O2) in the environment.

4. **Body's Response to Low Oxygen Levels**:
- To compensate for the lower availability of oxygen, the body adapts by increasing the production of red blood cells.
- More RBCs mean more hemoglobin, which is the protein responsible for carrying oxygen in the blood.

5. **Conclusion**:
- Therefore, the increase in the number of RBCs in individuals living at high altitudes is a physiological adaptation to ensure that enough oxygen is transported to tissues despite the lower oxygen availability in the atmosphere.

### Final Answer:
People living at high altitudes have around 8 million RBCs per cubic millimeter because the atmospheric oxygen level is lower at high altitudes, necessitating more RBCs to absorb the required amount of oxygen for survival.

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