The young couple have been looking for houses，but haven’t yet found they can affordA. one

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(答案→)A. one
B. that
C. them
D. it
解析：句意：年轻的夫妇一直在寻找房子，但是还没有发现他们能负担得起的房子。这里需要一个代词，指代a house，用one可以泛指可数名词的单数，that是特指可数名词的单数和不可数名词，them“他们”，it特指同类同物，所以选A。