After the initial cleaning of flue gas in an inert gas system the gas is passed through what device for final cleaning?（）.

A.pressure vacuum relief valves
B.excess flow valves
C.safety relief valves
D.flame screens

A.in the ends of each tank
B.in the expansion trunk of each tank
C.above the weather deck
D.there are no requirements in the regulation

A.phosgene gas
B.sulfuric acid gas
C.sulfur dioxide gas
D.carbon tetrachloride