既是气体的通道，又是食物的通道的是（    ）

根据短文内容，从A、B、C、D四个选项中 选择最佳答案填空。

    My name is Jim White. I 1 a schoolboy. My school is 2 my home. Every day, it takes me 3 to get there. The road is not 4. So I can't go to school 5. I often get there by bus or 6. It takes me thirty 7 to get there by bus and an hour on foot. I must get up very early every morning. I have no time 8 breakfast at home. I often have something for breakfast 9 or on the bus. I don't want to 10 school. So sometimes I run to school.

1996年美国通货膨胀率下降到2．1%，为30年来最低水平。1997年5月，美国失业率已降到4．8%，为1973年以来最低水平。1997年，美国经济增长率达3．2%，继续在西方国家中处于领先地位。出现这些现象的主要原因是（    ）