单选题 If the United States had built more homes for poor people in 1955, the housing problem now in some parts of the country______ so serious.
【正确答案】 C
【答案解析】本题主要考查虚拟语气。表示与现在事实相反的假设，虚拟语气的谓语需要用could(should, would等)+动词原形的形式。