题目

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 ex 下列各式中，值为的是 [　　] A． sin15°cos15° B． cos2－sin2 C． D．