题目

阅读短文，从方框中选择适当的动词，并用其适当形式填空。有的需要加助动词或不定式符号（每词限用一次)。 belong      pick      come      guard       choose There is a garden in front of a church(教堂) gate . Every day a number of children pass by the garden to go to school , so the flowers____1______ from time to time . One early morning, the pastor ( 牧师  ) stood in the garden  ____2______ the flowers. Soon , some children came along . A little boy asked the pastor , “ Can I pick a flower ?” “ Which one do you 9．用如图所示的滑轮组匀速提升重为500N的物体G，所用拉力F为200N，将重物提升3m所用的时间为10s，求：（1）做的有用功；（2）拉力的功率；（3）滑轮组的机械效率．