9.As developed countries vault forward on the back of new technologies, others will fall painfully farther behind.
(A) move quickly
(B) move slowly
(C) launch suddenly
(D) stop suddenly
艾游 高一下 (2015/06/04) 感恩樓上。另外查了一下: (轉)From Longman Dictionary Online: on the back ofsomething: as a result of something that already exists or something you have already done (www.english.com.tw/modules/newbb/viewtopic.php?post_id=12001)