干扰电疗时选用具有明显止痛的差频是（）
A.50～100Hz
B.20～40Hz
C.1～10Hz
D.0～100Hz
E.90～100Hz
参考答案：E
He _____ when the UFO arrived.
A. was sleeping B. were sleeping C. has slept
齿轮传动装置的作用是什么？
