—How long have you known the doctor ? —he helped me three years ago.A. When　B. Before　C. B

问题补充：

—How long have you known the doctor ?

—he helped me three years ago.

A. When

B. Before

C. Because

D. Since

答案：

A. When

B. Before

C. Because

(答案→)D. Since

解析：考查情景交际和时间状语从句。“你认识那个医生多久了？”“自从三年前他帮助我的时候。”since加含一般过去时的句子表示一段时间，主句用现在完成时。本句的完整形式是I have you known the doctor since he helped me three years ago.故选D。