—I'm sorry, but there's no smoking room in this section on the train.

—Oh, I _____ that, and I won't smoke again.   [ ]

A. don't know        

B. won't know      

C. didn't know      

D. haven't known

反映某一事件发生强度的指标应选用（）。

A.构成比

B.相对比

C.绝对数

D.率

E.变异系数