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阅读理解Parents may be surprised athow fast their kids grow up, but a study finds that U. S. teenagers aredeveloping more slowly than those in the past. Scientists at San DiegoState University have worked on a study of over 8 million U. S. kids aged 13-19for 40 years. The findings show that today's teenagers are less responsible(负责任的) and that most of them don't have a part-time job. The wholedevelopment has slowed down. Today's 18-year-olds are more like 15-year-olds inthe past, said Professor Jean Twenge. Teenagers are taking fewerrisks than those in the past. Scientists have al一根绳子所能承受的拉力是2.0×104N，能不能用它一次提起10块长2m、宽1m、厚1cm的钢板？（ρ=7.8×103kg/m3）