Give reasons :Mountaineers carry oxygen cylinders with them.

Explanation

### Step-by-Step Solution:

1. **Understanding Atmospheric Pressure**: At sea level, the atmospheric pressure is higher, which means the air is denser and contains a higher concentration of oxygen. As one ascends to higher altitudes, such as in mountainous regions, the atmospheric pressure decreases.

2. **Effect of Altitude on Air Density**: As altitude increases, the density of air decreases. This means that there are fewer air molecules, including oxygen molecules, in a given volume of air.

3. **Oxygen Availability**: Since oxygen makes up about 21% of the air, a decrease in air density at higher altitudes results in a lower volume of oxygen available for breathing. This can lead to insufficient oxygen supply for the body, which is crucial for maintaining energy levels and overall health.

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