单项选择题
Emulsion oils and sodium nitrite are both approved additives, but the latter cannot be used if any pipes are （） or if any soldered joints exit.
A.chromed B.welded C.galvanized D.bolted
单项选择题 Either a low injection pressure or a leakage of piston rings is able to lead （）.
单项选择题 Each piston stuffing box is provided with two sealing rings and three（）
单项选择题 Each journal and pin should be examined for （） and grooving.
