单项选择题
When a flash-type evaporator is to be secured for an extended period of time, the entire unit should be drained, cleaned, and （）.
A.completely filled with saltwater B.filled with de-scaling compound C.tightly closed to exclude air D.vented to the atmosphere
单项选择题 Condensate must be drained periodically from the air compressor receivers to prevent （）.
单项选择题 Overheating of a hydraulic system may be a result of （）.
单项选择题 The purpose of the low pressure cut-out switch is to （）.
