Have you got any particular plans for the coming holiday?

Yes. _____, I’m going to visit some homes for the old in the city.

A．If ever

B．If busy

C．if anything

D．if possible

肺泡与气体交换相适应的结构特点，除肺泡外包绕着丰富的毛细血管外，还有（　　）

A．肺泡数量较多

B．支气管呈树枝状分支

C．毛细血管中血液的流速很慢

D．肺泡壁和毛细血管壁都只由一层上皮细胞构成