适应于肺泡与外界空气之间进行气体交换的结构是( )
A:鼻腔内有鼻毛
B:气管、支气管都有软骨支架
C:肺泡壁有一层上皮细胞构成
D:肺泡外包绕着丰富的毛细血管
—Yes,I studied three.But I have forgotten all ____a few words of each.