Certainly I posted your letter—I remember it. A. posting B. to post C. to be posting D.

问题补充：

Certainly I posted your letter—I remember it.

A. posting

B. to post

C. to be posting

D. have posted

答案：

(答案→)A. posting

B. to post

C. to be posting

D. have posted

解析： remember doing sth=记住曾经做过的事，remember to do sth=记住要做的事。