单项选择题
Permission is kindly requested（）the ballast water.
A.pump out
B.to pump out
C.pumping out
D.be pumped out
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On a ship，a door that is required to be marked KEEP CLOSED is designed to（）.
A.prevent the passage of flammable gases
B.prevent the passage of poisonous vapors
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D.maintain watertight integrity -
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B.Gravity davits
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C.Gate valve
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