把直流电源中恒定的电压换成（）的装置称为直流斩波器。
A.交流电压
B.或调交流电压
C.脉动方波直流电压
D.可调直流电压
参考答案：D
( )要具备较强的安全意识，一旦发生事故能够采取必要的自救措施，有助于减少事故发生或降低事故损害程度。
A．乘客
B．乘务员
C．行人
D．驾驶员
33．China has produced many great writers over the last ten years, Mo Yan.
A．among them was
B．among whom was
C．in which was
D．among whom were
