—What's wrong with your coat?

—Just now when I wanted to get off the bus, the man next to me _________ on it. [ ]

A. is sitting

B. had sat

C. sits

D. was sitting

下列不属于滤料应满足的条件的一项是（）。

A.形状不规则

B.机械强度高

C.化学稳定性好

D.具有一定的大小和级配