单项选择题
A spark arrestor（）.
A.keeps sparks from falling into an open tank B.secures covers on ullage openings C.prevents sparks from getting out of an engine's exhaust system D.grounds static electricity
单项选择题 A relief valve for a cargo pump is generally installed（）.
单项选择题 A gas-free certificate would usually be issued by a(n)（）.
单项选择题 A fire of escaping liquefied flammable gas is best extinguished by（）.
