GK阀主活塞与铜套间隙大于（）时，更换主活塞或铜套。
A.0.4mm
B.0.5mm
C.0.6mm
D.0.7mm
参考答案：C
My colleague has been recovering gradually since the operation, and it wont’ be long ______ he comes back to work．
A．before
B．since
C．after
D．when
交流电是指（）随时间作周期性变化的电动势（电压或电流）。
A、大小
B、方向
C、大小和方向
