题目

With the rapid growth of population, the city _____ in all directions in the past five years. A. spreads      B. has spread         C. spread         D. had spread 答案：B 解析: 时间状语in the past five years与现在时间有关。下列哪项是肺泡适于气体交换的特点？（ ）?A．肺泡壁由一层上皮细胞构成B．肺泡数目很多，总面积很大C．肺泡外面缠绕着毛细血管D．以上三项都是