Last Sunday I saw the hardest storm in years. It came suddenly and went on for more than three hours. After lunch, I went into my room to have a rest. The air was hot, and all was quiet. Then the strong wind started blowing into my room. Pieces of paper on my desk flew high into the air and some flew out of the open window. As I ran out to catch them, big drops of rain began to fall. When I came back into the house, it was raining harder. I tried hard to close the window. Then I heard a loudcrashing sound from the back of the house. I ran out of my room to find out what it was--a big tree had fallen down and broken the top of the back room. The paper flew out of the window because of   _  .
the heavy rain the hot sun the strong wind the tall tree