单项选择题 Where tanks are arranged for either oil or ballast （） must be fitted in the pipe so that only the ballast main or the oil transfer main connected to the tank.

单项选择题 The switchboards are those in which all the live parts are installed behind the panels and only the operating handles and instruments are on the front（）

单项选择题 Modern centrifuge designs enable continuous over a considerable period of timeThis is achieved by which is timed to discharge the sludge at regular intervals（）