强雷暴发生时会出现（）
A.飑
B.冰雹
C.龙卷
参考答案：A, B, C
________ the working efficiency, the boss allows the employees to have a coffee break. [ ]
A. Improving
B. To improve
C. Having improved
D. Improved
门诊音叉试验最常用的音叉频率是（）
A.128Hz和256Hz
B.256Hz和512Hz
C.512Hz和1024Hz
D.512Hz和2048Hz
E.256Hz和2048Hz
