【名词&注释】
多级离心泵(multistage centrifugal pump)、往复泵(reciprocating pump)、安全阀(safety valve)、单螺杆泵(single screw pump)、齿轮泵(gear pump)、工作性质(nature of work)
[单选题]The water level in the gauge glass when a boiler is operating is _____ when the boiler is shut down.
A. lower than
B. higher than
C. below
D. equal to