单项选择题
Air that may enter a refrigeration plant tends to collect in the （）.
A.upper part of the receiver B.upper part of the condenser C.down stream end of the cooling coil D.outlet end of the condenser
单项选择题 Moisture should be kept out of an R-12 system because it tends to cause （）.
单项选择题 Black smoke coming from a boiler can be caused by （）.
单项选择题 If a bilge pump failed to build up discharge pressure, you should check for （）.
