题目内容：
根据下面材料，回答题。Some Schooling on Backpacks
According to the U.S. Consumer Product Safety Commission,5,900 kids were treated at hospital emergency rooms, clinics, and doctors offices' last year for sprains (扭伤) and strains caused by backpacks. Such injuries are so widespread that more than 70 percent of physicians surveyed by the American Academy of Orthopedic (整形外科的) Surgeons listed backpacks as a potential clinical problem for children.
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 tighten both straps firmly, so the pack rests about 2 inches above your waist.Also, remember to pack your bag with the heaviest 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 rule is not to exceed 20 percent of your body weight. So if a child weighs 100 pounds, the backpack and its load should not be more than 20 pounds. One hint: Make frequent trips to your locker (储物柜) to exchange books between classes.
Backpacks with wheels let you pull the weight along the ground, but they have problems too.
Many are larger than the average shoulder bag, so students are tempted to carry more than they would in a conventional pack.
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.
About six thousand American kids were injured'by carrying backpacks last year. A．Right
B．Wrong
C．Not mentioned
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