交－交变频器是利用自然换相，基于可逆整流原理把频率固定的三相交流电直接变换成频率连续可调的三相交流电。
参考答案：对
A car accident ________ him last week. [ ]
A. took place to
B. happened on
C. happened to
D. happens to
脂溶性最高的药物是
A．苯巴比妥
B．硫喷妥钠
C．戊巴比妥
D．司可巴比妥
E．异戊巴比妥
