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根据短文内容，选择正确答案。

    Tom is a junior high student. He is good at some subjects but his favourite subject is chemistry. Yesterday he met some trouble when he had his chemistry class.

    At that moment, Tom was doing an experiment in class. He tried twenty times, but he couldn't succeed. So he decided to stop doing the experiment. He felt really unhappy.

    His teacher came up to him and asked why he was unhappy. Tom said, "All my work is in vain (白费力气). I fail twenty times and I get nothing."

    Tom's teacher touched Tom's head with a smile. He said to Tom, "Oh, you do a lot and you can get a lot from the mistakes. At least, you know that there are twenty ways you can't use to do this experiment."

    After hearing the teacher's words, Tom became excited. He kept doing the experiment, improved his methods, and he succeeded at last.

贮藏粮食时，保持干燥和低温是为了降低（　　）