单项选择题 The rising velocity of the globules carries them upwards where they become trapped by the under surfaces of plates and （）until the enlarged globules have sufficient rising velocity to travel along the plate surface and break away at periphery.

单项选择题 Humans are still needed to monitor the condition of the boiler all the time the latter is operated, （）it is just to acknowledge an alarm buzzer or flashing light in a control room.

单项选择题 The steam boiler is part of a closed loop steam, whereby, after the energy of the steam is used up and it becomes water again, （） is collected and returned back to the boiler for heating up again to steam.