单项选择题
A.once a week
B.once a month
C.once a year
D.twice a year
单项选择题 For safety reasons，when assistance towing（）.
单项选择题 For pumping the bilges，a 54 foot long “T-Boat”，which is not a ferry，but is certificated to carry 30 passengers，must be fitted with at least（）.
单项选择题 For optimum stability during jacking operations，mat ballast tanks or spud cans should be（）.