单项选择题 The water level in the gauge glass when a boiler is operating is （） when the boiler is shut down.

单项选择题 Large pipes or （） are fitted between the steam and water drums to ensure good natural circulation of water.

单项选择题 （） system is arranged to ensure that water can be drawn from any tank or the sea and discharged to any other tank or the sea as required to trim the vessel.