Chọn một phương án A, B, C, hoặc D để điền vào chỗ trống trong văn bản sau. 
Five hundred years ago in the USA, teenagers mostly worked and learned to work. Boys learned to Most teenagers didn't go to school (17)_______the lack of schools and the inability of parents to pay for education. 
One hundred years ago, many 13- and 14-year-olds went to school since it was free until they turned grow food or care for animals from their fathers, while girls learned household tasks from their mothers 15. However, most left to work in factories or on farms. Life was hard for teenagers, especially in poor areas, but some (18)_______dance halls with music or new movie theaters. 
Fifty years ago, in the 1970s, teenagers, (19)_______gained more freedom than ever, experienced significant changes in their lives. Many attended school until they were 16, and when not in school, they enjoyed life instead of working in difficult jobs. Teenagers began to (20)_______their own identity, not just as older children. This trend has continued to grow. 
Today, life is very different for 13, 14, or 15-year-olds in the USA compared to the past. 
17. A. because of     B. and                C. although      D. but 
18. A. enjoyable       B enjoyment       C. enjoyed       D. enjoyably 
19. A. when              B. which             C. whom          D. who 
20. A. prepare          B. develop          C. improve       D. create