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

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

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。

这个问题的回答的对