听力原文： 'What do you think is really important in the United States? I really think that independence is one of the most important American values. From childhood we learn to be independent, to think for ourselves, to stand on our own two feet. American society allows its young people to move away form. their families at eighteen or nineteen years of age. Of course, not all young people leave home at eighteen, nor do most parents want their children to leave. However, living independently from one's family is an accepted part of the American value system. 15. Why do American young people usually leave their families at eighteen or nineteen?A. Their working place is too far away from their families. What does the passage mainly discuss?