问题补充：
I became a teacher ten years . I have been a teacher .
A. ago; for
B. before; since
C. ago; since
D. ago; in
答案：
A. ago; for
B. before; since
(答案→)C. ago; since
D. ago; in
解析：句意：我十年前成为一名教师，我从就是名教师了。第一句用一般过去时，通常和ago连用；第二句用的是现在完成时，可与since连用，自从……。
