单项选择题

救生艇下水每隔三个月进行一次。（）

A. Lifeboat launching is operated at intervals of three months.
B. Lifeboat launching is operated at intervals for three months.
C. Fire drill is carried out once a month.
D. Don’t operate two or more hatch covers at the same time.

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Which letter means that oil-spilling has happened?（）

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（）is best for electrical fire.

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C.A+B
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