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，从短文后所给各题的四个选项 (A、B、C和D) 中
选出能填入相应空白处的最佳选项。
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．
21．A．beneficial B．friendly C．tiring D．unhealthy
22．A．made B．took C．had D．brought
23．A．harmed B．helped C．improved D．reduced
24．A．aimed B．connected C．indicated D．represented
25．A．directed B．driven C．hung D．placed
26．A．clean B．clear C．proper D．visible
27．A．breaking B．failing C．letting D．slowing
28．A．body B．condition C．situation D．state
29．A．appreciate B．assist C．promote D．support
30．A．bridge B．means C．proposal D．warning