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（）.

单项选择题 An oil tanker operating with dedicated clean ballast tanks shall be equipped with（），approved by the Administration on the basis of specification recommended by the Organization，to enable supervision of the oil content in ballast water being discharged.

单项选择题 Air gap is the vertical distance between the bottom of the hull and the（）.