电动机的调速有（1）（）；（2）（）；（3）（）：（4）变频调速等方法。
参考答案：调节电源电压；改变转差率；改变磁极对数
普通X线摄影用的管电压范围是（）
A.25～40kVp
B.35～40kVp
C.40～60kVp
D.40～100kVp
E.40～150kVp
—Li Dong, are you going to the cinema tonight?
—Yes, I must go. It does _______ a few good features in this movie. [ ]
A. have
B. has
C. to have
D. had
