单项选择题
A.in boiler drums
B.in refrigerating plants
C.in starting air pipes
D.in air coolers
单项选择题 Air reservoirs are designed, built and tested similarly to （）.
单项选择题 The air reservoirs must have （） fitted to prevent rupture in the event of fire.
单项选择题 Before liquid can flow into a pump, the air in the suction line must be （） sufficiently to cause the liquid to flow into the suction chamber.