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      Not only adults but also teenagers have problems in their life. Here is a survey showing the main problems of teenagers.

　　They feel stressed because they have too much homework to do both at school and at home. They have lots of exams to take. And parents usually send them to different classes at weekends. They don't have their own time to do what they are interested in.

　　Now more and more teenagers are getting short­sighted. They often read in bed or keep reading for a long time without having a rest. Some of them are crazy about playing computer games. Some spend too much time watching TV.

　　Another serious problem among teenagers is that many of them are becoming fat. They eat too much junk food, but they take little exercise.

　　I think teenagers should think of ways to deal with problems. They should make a plan for study and hobbies and find time to relax as much as possible.

“节能”是建设节约型社会的要求。由于人们观念上的差异，人们对墙体和屋面的保温性能长期以来并不关注，不少地方节能型房屋的推广进展缓慢。这告诉我们（   ）

要配制100g溶质质量分数为10％的氯化钠溶液，下列操作中正确的是（）