The Young Pioneers didn’t know       .

A．where will they go

B．what they would do next

C．when could they go

D．how did they get there

晶闸管工作时，只有导通和阻断两种状态，因此可用它制成（）开关。

A.直流无触点

B.直流有触点

C.交流无触点

D.交流有触点