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， 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若，则代数式的值为（ ）A. B. C. D.