Question

He had already walked three or four miles ________ he saw a cart, half-full of hay, by the side of the road．

1. A.
   while
2. B.
   when
3. C.
   the moment
4. D.
   unless

Answer 1

B
解析：when“這時(突然)”。常見搭配：be doing…when；be about to do…when；had done…when
拓展：例如：I was walking in the street when someone called me from behind．我走在街上，突然有人從后邊喊我。I was about to go out when he came in．我正要出門，突然他進(jìn)來了。
I had hardly finished bathing when the electricity went off．我剛洗完澡，就停電了。