声音是由发声体的_______而产生的，在固体、液体和气体中，传播声的速度最慢的是________。
振动；空气
I won't tell the student the answer to the math problem until he____ on it for more than an hour. [ ]
A. has been working
B. will have worked
C. will have been working
D. had worked
为达到治疗时兴奋A类纤维，激活粗纤维，TENS治疗仪所设计的脉冲波宽为()
A．0.04～0.3ms
B．0.3～0.4ms
C．0.4～0.45ms
D．0.45～0.5ms
E．0.01～0.04ms
