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 eat more nutritive food, therefore more RBCs are formed
B. people get pollution-free air to breathe and more oxygen is available
C. atmospheric `O_(2)` level is less and hence more `RBCs` are needed to abosorb the required amount of `O_(2)` to survive
D. there is more UV radiation which enhances RBC production.

Explanation

### Step-by-Step Solution:

1. **Understanding RBC Count**: Red Blood Cells (RBCs) are crucial for transporting oxygen in the blood. The normal count at sea level is about 5 million RBCs per cubic millimetre.

2. **Altitude Effect**: At high altitudes, such as 5400 meters, the atmospheric pressure decreases, leading to a lower density of air. This means that there is less oxygen available in the air.

3. **Body's Adaptation**: To compensate for the reduced oxygen availability at high altitudes, the body increases the production of RBCs. This is a physiological adaptation to ensure that enough oxygen can be transported to tissues and organs.

4. **RBC Count at High Altitude**: As a result of this adaptation, people living at high altitudes have a higher RBC count, approximately 8 million per cubic millimetre, compared to those at sea level.

5. **Conclusion**: The increase in RBC count in individuals living at high altitudes is primarily due to the lower atmospheric oxygen levels, which necessitate more RBCs to absorb the required amount of oxygen for survival.

### Final Answer:
The reason people living at high altitudes have more RBCs is that atmospheric O2 levels are less, and hence more RBCs are needed to absorb the required amount of O2 to survive.

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