—I fell and hurt my leg just not. —_______.

A．Be careful

B．It doesn’t matter

C．I’m sorry to hear that

D．Nothing serious

肺泡与进行气体交换相适应的特点有：肺泡数量   ，肺泡外面包绕着丰富的            ，肺泡壁与毛细血管壁都很   ，是   层扁平的上皮细胞。