单项选择题
A.year
B.two years
C.four years
D.five years
单项选择题 By regulation，cargo tanks must be inert before and during what operation?（）.
单项选择题 By regulation，cargo tank atmosphere must be inert before and during which operation?（）
单项选择题 An oil tanker，except as provided for otherwise，might discharge certain amount at certain rate of oil or oily mixtures into sea when（）.