冷柜压缩机运转正常，但高低压均低于正常值，此时应检查（）。
参考答案：系统是否泄漏
The number of historic buildings which ________ has reached a surprising figure that we must be aware of.
A．has been pulled down
B．had been pulled down
C．is being pulled down
D．have been pulled down
系统抽残液用的泵一般都采用往复泵，是因为（）的原因。
A、压头高
B、流量大
C、不怕抽空
D、操作简便
