A train stops at a station(车站). A young man wants to come out, but it is raining. A boy is standing under a big umbrella. The young man says to the boy，"Can you go and get us two hamburgers, one for you and one for me? Here are two dollars. " "Great！"says the boy and he goes to buy hamburgers. After some time, the boy is back. He is eating a hamburger. "Where is my hamburger？"asks the young man. "Oh, there is only one hamburger left(剩下). So I'm eating mine. Here is your dollar. "

下列划线字注音正确的一项是（  ）。

图为肺的内部结构示意图，图中所示肺泡外包围着毛细血管，肺泡壁和毛细血管壁都很薄，由一层上皮细胞构成，这有利于（  ）