The correct answer is: B. nitrogen contents
Rainwater is a natural source of nitrogen, which is an essential nutrient for plant growth. Nitrogen is found in the air, and it is also released into the atmosphere when plants and animals decompose. When it rains, the nitrogen in the air and in the soil is dissolved in the rainwater. This nitrogen-rich rainwater then falls on plants, providing them with a valuable source of nutrients.
The other options are incorrect because:
- Phosphorus is not a major component of rainwater.
- Calcium is a major component of limestone, which is not dissolved in rainwater.
- Potash is a major component of potassium salts, which are also not dissolved in rainwater.
In addition to nitrogen, rainwater also contains other nutrients that are essential for plant growth, such as phosphorus, potassium, and calcium. However, the amount of these nutrients in rainwater is typically much lower than the amount of nitrogen.