单项选择题 When to remove scale from the cooling spaces, acid solution （） in the system can be tested from time to time by putting some on to a piece of lime.

单项选择题 When the water produced from most evaporator is supplied for drinking purpose, it must be treated to ensure it is biologically pure and （）.

单项选择题 When the starting air valve closes on one cylinder, another starting air valve has （） opened on another cylinder whose piston has just commenced its downward stroke.