可控硅只有在（）加正向电压和控制要加适正向控制电压的情况下才能导通。
A、阳极
B、阴极
C、控制极
D、触发极
参考答案：A
Put the following events_______ according to the time when they happened and then putthem
______ of their importance in history. [ ]
A. in order; in order
B. in the order; in an order
C. in an order; in order
D. in order; in order
检验检疫证书的法律效用主要体现在以下几个方面：______。
A．是出入境货物通关的重要凭证 B．是海关征收和减免关税的有效凭证C．是履行交接、结算及进口国准入的有效证件 D．是议付货款的有效证件
