单项选择题
If no gaskets are used in the piping joints of a Freon system, the joints must be（）
A.finished joints B.welded joints C.ground joints D.soldered joints
单项选择题 If the temperature in the icebox is too high, the trouble could be（）
单项选择题 Air or noncondensable gas is removed from a refrigeration system by a （）
单项选择题 Where are the zinc plates located?（）
