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

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

D. Since

解析： 考查连词辨析。A 当。时候；B 在之前；C 因为D 自从。句意 你认识这个医生多久了。自己三年前他帮我开始。上文提到现在完成，与since引导的一般过去时相呼应。故选D项。