An obstruction on a helodeck is any object that might present a hazard to the（）.

A.prevent the generation of flammable or combustible gas in tanks
B.blow out cargo lines to prevent the build up of gas concentrations
C.dilute tank atmospheres to keep gas concentrations below the lower explosive limit
D.prevent fires in the pumproom by continually displacing flammable vapors

A.aid in the stripping and cleaning of cargo tanks
B.increase the rate of discharge of cargo
C.force toxic and explosive fumes from a cargo tank to vent to the outside atmosphere
D.lower the oxygen levels inside cargo tanks，making explosion nearly impossible

A.with
B.has
C.have
D.had