单项选择题
A.a centrifugal fan
B.a direct expansion cooler
C.a hermetic compressor
D.a centrifugal pump
单项选择题 With ducted air conditioning systems, the modern tendency is to use（）
单项选择题 The（） only allows for adjustment of temperature in each room by the occupant manually controlling the air volume admitted.
单项选择题 The volute of centrifugal pump also acts as a diffuser converting （）into pressure head.