单项选择题
A centrifugal impeller consists of a number of vanes curving （） from the direction of rotation.
A.backwards B.forwards C.upward D.downwards
单项选择题 When installed for bilge pumping or ballast duty, centrifugal pumps require a （） to remove air from the suction pipe.
单项选择题 Water enters the rotating impeller through the entry, known as “（）”, at the centre.
单项选择题 Which one is not the normal operation for boiler shutdown?（）
