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.

单项选择题 Care must be taken to condition the （）in the boiler to prevent scaling, and corrosion, which can weaken the pressure vessel.