He had already walked three or four miles ________ he saw a cart, half-full of hay, by the side of the road．A.whileB.whenC.the momentD.unless
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B解析解析：when“这时(突然)”。常见搭配：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，突然有人从后边喊我。I was about to go out when he came in．我正要出门，突然他进来了。I had hardly finished bathing when the electricity went off．我刚洗完澡，就停电了。