秋天，稻子“笑弯了腰”，说明力可以改变物体的____．跳高场上，运动员“腾空而起”，说明力可以改变物体的____ ．

根据短文理解，从A、B、C三个选项中选择最佳答案填空。

    A young father was visiting an old man. They were standing in the old man's garden and  1  children. The young man said, "How strict should parents be  2their children?" The old man pointed to a line3 a big, strong tree and a thin, 4  one. "Please untie the line," he said. The young man 5 so, and the young tree bent(弯曲) over to one side.

    "Now tie it again," said the old man, "but first pull the line tight(紧紧地) so that the young tree is6  again." The young man did as the old man told him. Then the old man said, "It is the same with  7  . You must be strict with them, but sometimes you must untie the 1ine8 how they are getting on. If they are not yet able to stand alone, you must tie the line tight 9 . But when you find that they are ready to stand alone, you can take the10   away."

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