1)One of the greatest problems facing the 21st century will be what to do with all the rubbish produced in the world.
2) The frequent use of drugs (毒品) is a serious problem among some American young people.
3) To travel abroad is a desire shared by many people, but few can afford to do it.
4) Man still has a lot to learn about the most powerful and complex part of his body-the brain.
5) We had a ling debate as to whether we should spend so much money on space technology.
6) Many people have the bad habit of spending money on impulse.
7) The custom of having one’s marriage arranged by parents has disappeared in many parts of the world but continues in some Asian and African countries.
8) Cheating is now considered to be a major problem in colleges and universities.
9) No agreement was reached as to how the plan should be carried out.
10) True, the cost of living has gone up, but so have the workers ‘wages.
11) I had trouble learning or doing something important to me.
12) She had a little trouble with her typing.
