题目

The curtain on the 2008 US presidential election finally rose last month as John McCain and Barack Obama were formally nominated （提名） as candidates of the two major parties. This may be one of the hardest decisions voters have to make between two appealing candidates. The big question for voters, as they face both an economic downturn and international threats, is: who will they elect? A young first-term senator promising change and new ideas, or a longtime senator with strong military experience and a reputation as a maverick （特立独行的人）? American voters have never seen 若右图中 a、b、c、d表示的是城市工业、商业、 住宅和蔬菜用地的土地付租能力与市中心距离的关系， 则表示住宅用地付租能力线的是： A．a                B．b C．c               D．d