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。