^-^老师请帮一下.

2. I haven’t heard from Bill_____________ he left here.

　　A. when B. before C. since D. whileA.
我认为when 和. since都可以啊 但答案说是since ?

A. when 是延续性吗?\
的确答案要好点,但当时做的时候脑子都不动就选了

看来你已经懂本题为什么选C了。给你说下这两个词的用法：
since 连接词 conj.

1.(常用于现在完成时或过去完成时)自...以来,从...至今
It's been ten years since they married.
他们结婚到现在已经十年了。
She has been living a hard life since her husband died.
她自从丈夫死后,一直过着艰苦的生活。
2.既然;因为,由于
He must have taken the book since it isn't here.
他一定是把书拿走了,因为书已不在这里了。
Since it is so hot, let's go swimming.
既然天气这么热,我们去游泳吧。
本句是第一种用法。

when连接词 conj.

1.当...时
John got engaged to her when travelling last winter.
去年冬天旅行时,约翰与她订了婚。
Don't get excited when you talk.
你说话时别激动。
2.既然,考虑到
How can he get the promotion when his boss dislikes him?
既然上司不喜欢他,那他怎么会获得提升呢?
3.虽然,然而
I had only fifteen dollars on me when the book I wanted cost twenty.
我身上只带了十五元钱,可是我想买的那本书定价二十元。
4.在那时,然后