—How old are you?
—I'm fifteen. I was born _____ 1990. [ ]
A. in
B. at
C. on
D. for
答案:A
在正交偏光镜下,2个非均质体切片相互重叠时,异名轴平行,光透过两切片后,若(),此时切片消色而出现补偿黑带。
简述YJ19卷烟机磨刀装置的保养。