“It,s hard for me to get up the

　　1

　　with my bag because its so heavy,”said Rick Hammond,

　　2

　　11yearold student in the US.

　　Rick is among the students who have

　　3

　　backpacks（背包）with two straps（带子）to carry them,

　　4

　　a number of other students choose

　　rolling backpacks.

　　But even with rolling backpacks,

　　5

　　up stairs and buses with them is

　　6

　　a problem for kids.Many of them have hurt their knees,backs or necks because of heavy school bags.

　　But how much is too

　　7

　　?Experts say students should carry

　　8

　　more than 10 to 15 percent of their own body weight.

　　Scott Bautch,a Wisconsin

　　9

　　doctor,said kids under 4th grade should

　　10

　　with 10 percent.But it,s also important that older kids don,t go

　　11

　　percent,because their bones are still growing.

　　Bautch explained that there are other injuries caused by backpacks.“Kids are

　　12

　　their balance and falling down with these backpacks,”he said.

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