当机泵发生汽蚀时，下列说法错误的是（）。
A、泵内有响声
B、流量和压力增高
C、泵体振动较大
D、造成抽空
参考答案：B
--- I ______ so busily recently that I ______ no time to help you with your math.
--- That’s OK. I can manage it by myself.
A．have been working; have
B．have worked; had
C．am working; will have
D．had been working; had had
光镜结构常用计量单位是：（）。超微结构常用计量单位是：（）。
A.μm
B.mm
C.nm
D.cm
