血液流经肺部毛细血管时，氧气由肺泡进入血液的原因是（    ）

根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

    Tom with his family 1 near the sea. Their house is not very high, but it is beautiful. There 2 big trees and a small garden in front 3 their house. They plant flowers in the garden. Some of the flowers are red, some are white 4 some are blue. Tom and his family love 5 garden very much.

      6 Tom and his sister go to school 7 bus. The bus is comfortable. There are air conditioners in the bus. And it is a double-decker bus. After school, they sometimes help their mum 8 some shopping at the shopping centre, and sometimes help their dad in the garden. At weekends, they often take some photos by the sea. 9 summer, they often go swimming 10 their parents.