单项选择题
Moisture should be kept out of an R-12 system because it tends to cause （）.
A.icing of the automatic controls B.embrittlement of some parts of the automatic controls C.sludge in the lubricating oil D.all of the above
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 （）.
单项选择题 The shaft and spare parts（） should be well greased in order to avoid corrosion.
