单项选择题
When checking the oxygen content of the cargo tanks prior to loading cargoes requiring vapor recovery，the atmosphere must be sampled（）.
A.one meter from the tank bottom and one meter below the tank top
B.one half the ullage of the tank and one meter below the tank top
C.one half the ullage of the tank and one meter above the tank bottom
D.at three meter intervals from the tank top
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单项选择题
When checking the oxygen content of the cargo tanks prior to loading cargoes requiring vapor recovery，the atmosphere must be sampled（）.
A.one meter from the tank bottom and one meter below the tank top
B.one half the ullage of the tank and one meter below the tank top
C.one half the ullage of the tank and one meter above the tank bottom
D.at three meter intervals from the tank top
