单项选择题
A.closed, and no leakage permitted across the shaft packing
B.closed, and only slight leakage permitted across the shaft packing
C.open, and no leakage permitted across the shaft packing
D.open, and only slight leakage permitted across the shaft packing
单项选择题 Sub-cooling is a method of reducing the temperature of the liquid refrigerant below its （）.
单项选择题 If a bilge pump is able to develop vacuum, but is unable to sufficiently pump out the bilges, you would check for all of the following EXCEPT （）.
单项选择题 The principal purpose of sub-cooling liquid refrigerant prior to its entering the expansion valve is to （）.