—How long have you known the doctor ?
— he helped me three years ago.A.WhenB.BeforeC.BecauseD.Since
网友回答
D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。