单项选择题

Some Schooling on Backpacks
According to the U.S. Consumer Product Safety Commission, 5,900 kids were treated at hospitalemergency rooms, clinics, and doctors' offices last year for sprains ( 扭伤 ) and strains caused by backpacks.Such injures are so widespread that more than 70 percent of physicians as a potential clinical problem forchildren. How do you avoid such problems? Choose bags that have wide, padded straps (有垫的背带 ) and a belt.That will help transfer some of the weight from the back and shoulders to the hips. You should also tightenboth straps firmly, so the pack rests about 2 inches above your waist. Also, remember to pack your bag with theheaviest items closest to your back and to bend both knees when you pick it up. How much should you stuff into your pack? That depends on your size and strength, but a general ruleis not to exceed 20 percent of your body weight. So if a child weights 100 pounds, the backpack and its loadshould not be more than 20 pounds. One hint: Make frequent trips to your locker ( 储物柜) to exchange booksbetween classes. Backpacks with wheels let you pull the weight along the ground, but they have problems too. Many arelarger than the average shoulder bag, so students are tempted to carry more than they would in a conventionalpack. Roller bags often don't fit into a locker. They can also lead to tripping and falls in crowded halls.Whatever you use, 10 or 15 minutes of stretching and back strengthening is a good idea.

A 10-15 minutes’exercise will help you bear a heavier backpack.（）

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