第一节：完形填空（共10小题；每小题2分，满分20分）
阅读下面短文，掌握其大意，然后从21～30各题所给的四个选项（A、B、C和D）中，选出最佳选项，并在答题卡上将该项涂黑。
Exercise is often said to be good for everyone．But a new study has shown that exercise could be _21 especially for people past middle age．The study was done with mice．It showed that although mild exercise 22 a good effect on the muscles of young mice, it 23_ the muscles of mice past middle age．
The three groups of mice in the experiment were _24__ in age at young, middle-aged, and elderly humans．The mice were __25_ on a thread-mill (踏车) 30 minutes a day for a five-week period．At the end of that time, the young mice’s leg muscles had increased almost 30 percent in weight, but those of the old mice were reduced by 25 percent．No_ 26_ pattern showed for the middle-aged mice．
David Gershon, an expert who carried out the experiment, explained that the older mice’ muscles were probably already _27_ down and that exercise worsened the _28_ ．An earlier study by Gershon showed that middle-aged mice benefited from exercise if they started exercising young and continued it．
It’s clear that because the tests were done on mice, drawing conclusions about humans is too early．But the studies do not totally _29_ the theory that exercise is good under any conditions．Let’s use the tests on mice as a _30__for other studies．
