加氢精制的作用，主要是去掉原料中的硫，氮，氧，使不饱和烃（）。
A.减少
B.饱合
C.增加
D.不变
参考答案：B
简述问题行为的性质。
It's an old photo of my father.I________it when I was looking for my passport the other day.[ ]
A. came into
B. came about
C. came across
D. came on
