The correct answer is: D. Humidity
Humidity is the amount of water vapor in the air. It is measured as a percentage of the total amount of water vapor that the air could hold at a given temperature.
Water vapor is a gas that is invisible to the naked eye. It is formed when water molecules evaporate from liquid water or ice. The amount of water vapor that the air can hold depends on the temperature of the air. Warmer air can hold more water vapor than cooler air.
When the air is saturated, it is holding as much water vapor as it can at that temperature. If the air is not saturated, it is holding less water vapor than it could.
Humidity is important because it affects the way we feel. When the humidity is high, we feel sticky and uncomfortable. When the humidity is low, we feel dry and uncomfortable.
Humidity also affects the way our bodies regulate temperature. When the humidity is high, it is harder for our bodies to cool down. When the humidity is low, it is harder for our bodies to warm up.
Humidity also affects the way plants grow. Plants need water to grow, but too much water can damage their roots. The ideal humidity for plants is between 50% and 70%.
Humidity is also important for the environment. It helps to regulate the Earth’s temperature and it helps to distribute water.