变电站内防误闭锁装置应（）进行一次试验。
A、每月
B、每季
C、每半年
D、每年
参考答案：C
I don't remember where _____ in that city when he was young? [ ]
A. did he work
B. he work
C. he has worked
D. he worked
生产经营成本高，大多是不盈利的是( )期的价格策略。
A．导人期
B．成长期
C．成熟期
D．衰退期
