How much more expensive do you think a flat in a downtown area is than in a suburban area

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(答案→)A. one
B. the one
C. that
D. those
解析：句意：你认为一个商业中心的公寓要比郊区的公寓贵多少？这里需要代词代替a flat，可以用one泛指前面的可数名词单数，the one是特指可数名词单数，that是特指可数名词单数或不可数名词，those是特指可数名词复数，所以选A。