Weather changes when the temperature and the amount of water in the atmosphere change. We can see and feel water coming from the atmosphere when we have rain. But the water must somehow get back to the atmosphere. Meteorologists call this the water cycle.There are many stages in the water cycle. Rain falls when water vapor in clouds condenses (凝结). Drops of water form and fall to the ground. The water soaks into the ground and feeds streams and rivers. A lot of rain falls into the sea. The heat of the sun evaporates some of the water in the ground and in the rivers, lakes, and the sea. It changes the liquid water into water vapor. The vapor rises onto the air. Water vapor is normally invisible. On a very damp of humid day, however, you can sometimes see water vapor rising from a puddle (水坑) or pond in a mist (薄雾) above the water. Water vapor also gets into the air from living things. Trees and other plants take in water through their roots and give off water vapor from their leaves. People and land animal drink water and breathe out water vapor. In all these ways the water returns to the air. There it gathers to form clouds and condenses to form rain. The rain falls to earth, and the cycle starts again. It continues even if snow or hail (冰雹) fall instead because both eventually melt to form water. The amount of water vapor in the air depends on the temperature. The air is more moist (潮湿) in the tropics (热带) than in the cold polar regions. What is the main idea of the passage()

A. Water cycle.
B. Water vapor.
C. How rain forms
D. Water, vapor, rain.