2016年4月，******指出，“一带一路”建设是我国在新的历史条件下实行全方位对外开放的重大举措、推行互利共赢的重要平台。其中的“一带”是指“丝绸之路经济带”。在宋元时期，丝绸之路是（    ）

    I am a student. I like sports. I run every morning and I swim twice a week(一周两次). I like all kinds of ball games. I like football, basketball, volleyball, tennis and ping­pong. I am good at playing ping­pong. I often play ping­pong with my friend, Li Lei. We are on the school team. But he doesn't like any other ball game. He also likes to collect(收集)balls. He has a great collection(收藏品)of balls. He has 60 footballs, 16 volleyballs and many ping­pong balls. We like to watch sports on TV.

肺泡处气体交换后血液的变化是（　　）