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