— What made you so sad?
— _____ my wallet. [ ]
A. Lose
B. Losing
C. To lose
D. Lost
答案:B
一条光线垂直射到平面镜上,入射角是______,如果将光线向镜面的方向转动30°角,这时反射角为______.
泵体振动的原因是什么?