下列实例属于防范物体惯性的是（　　）

    Most American families are smaller than the families in other countries. Children in the USA will leave their parents when they grow up. They usually live far from their parents because they want to find good jobs. They often write to their parents or telephone them. And they often go to visit their parents on holidays.

    Parents usually let their children choose their own jobs. Americans think it is important(重要的) for them to make decisions(作决定).

    Children are also asked to do some housework at home. And in many families，children will get money from their parents for doing some housework. Because American parents want to let their children learn how to make money for their own use.

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Many years ago， there lived a very rich man who wanted to do something good for the people of his town. But first he wanted to find out if they deserved(值得)his help. So he put a very large stone in the middle of the main road into the town. Then he stood behind a tree waiting and watching.

Soon an old man came along with his cow. “Who put this stone in the center of the road?” said the old man， but he didn’t try to move the stone away. He passed around the stone with some difficulties and continued on his way. Another man came along and did the same thing；then another came and another. All of them complained about the stone in the center of the road but nobody tried to move it.

Late in the afternoon a young man came along. He saw the stone and said, “It will be dark at night. Some people will come along later in the dark and will fall over the stone and get hurt.” The young man then began to move the stone. He pushed and pulled and tried everything he could to move it out of the road. But to his surprise， under the stone he found a bag full of money and this message, “This money is for the person who moves this stone from the road. The person deserves help.”