题目

“若x2＝1，则x＝1”的否命题为( ) A．若x2≠1，则x＝1                 B．若x2＝1，则x≠1 C．若x2≠1，则x≠1                 D．若x≠1，则x2≠1 答案：C() 8. Beijing a lot in the past tenyears. Now it has become one of the biggest modern in the world.   A. changes; cities   B. changed; city   C. has changed; cities   D. changed; cities