单项选择题 Impurities from the boiler water surface can be removed through the （）.

单项选择题 All bilge suction must be fitted with suitable （）, which in the machinery space would be mud boxes positioned at floor-plate level for easy access.

单项选择题 Centrifugal pumps, however, can be designed with a number of impellers （） to attain a large amount of liquid.