单选题What is TRUE of pressure/vacuum valves? （）AThey are designed to provide for the flow of small volumes of tank atmospheres caused by thermal variations in a cargo tankBThey should operate in advance of the pressure/vacuum breakersCThey should be kept in good working order by regular inspection and cleaningDAll of the above
单选题
What is TRUE of pressure/vacuum valves? （）
A
They are designed to provide for the flow of small volumes of tank atmospheres caused by thermal variations in a cargo tank
B
They should operate in advance of the pressure/vacuum breakers
C
They should be kept in good working order by regular inspection and cleaning
D
All of the above
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