A abrupt adj.生硬的；突然的；唐突的；陡峭的 describes something that is sudden and unexpected,and often unpleasant
B.quick adj.快的；迅速的,敏捷的；灵敏的 happening or done with great speed,or lasting only a short time
C.Immediate adj.立即的；直接的；最接近的 happening or done without delay or very soon after something else
D.instant adj.立即的；紧急的；紧迫的 happening immediately,without any delay
从many passengers fall off their seats判断,火车是在人们毫无思想准备的情况下突然停的,所以应选A：
1.The train came to an abrupt stop ,making many passengers fall off their seats.
火车突然忽然一下子停了,很多乘客都从座位上跌落下来.