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.

单项选择题 When the order （） is received from the bridge, the main air stop valve in the starting air line should be shut.