Explanation:

When it rains, the air becomes humid, which means that it contains a high amount of water vapor. Our bodies have a natural cooling system in place, and one of the ways we cool down is by sweating.

During rainy season, the air is already saturated with moisture, and our sweat takes longer to evaporate because there is already a high amount of moisture in the air. This is why we tend to perspire more during rainy season.

Here is a detailed explanation:

1. The presence of water vapor in the air:
- Water vapor is the gaseous form of water that is present in the atmosphere.
- During the rainy season, the air becomes more humid because of the high amount of water vapor present.
- When the air is humid, our sweat takes longer to evaporate into the air because there is already a high amount of moisture in the atmosphere.
- This leads to a buildup of sweat on our skin, resulting in perspiration.

2. The role of perspiration in cooling our bodies:
- Perspiration is the process of sweating, where our sweat glands produce sweat that is released onto the surface of our skin.
- When the sweat evaporates from our skin, it takes away heat from our bodies, cooling us down.
- However, during the rainy season, the high amount of water vapor in the air hinders the evaporation of sweat, making it more difficult for our bodies to cool down efficiently.

Therefore, the correct answer is option B) Water vapor. During the rainy season, the presence of water vapor in the air hinders the evaporation of sweat, causing us to perspire more.