车间净化系统的动力源是（）。
A、压缩空气
B、高压离心风机
C、罗茨风机
D、排烟风机
参考答案：D
– You look very tired this morning. What did you do last afternoon?
-- I did _______ Christmas shopping.
A．a number of
B．a few of
C．a lot of
D．a piece of
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