Spending two or three hours playing outdoors each day can reduce(减少)a child’s chance of becoming short-sighted（近视）,research shows . It challenges the belif that short-sightedness is caused by computer use , watching TV or reading in weak light . Australian government researchers believe that sunlight is good for people’s eyes . They compared the vision （视力）and habits of 100 seven-year-old children in China and Australia .In all,30% of the Chinese children were short-sighted .This percentage was ten times higher than Australian children. Both groups spent a similar amount of time reading, watching TV and playing computer games .However , the Australia children spent an average of two hours a day outdoors ,90 minutes more than the Chinese children. Professor Ian Morgan said ,“Humans are naturally long-sighted ,but when people begin to go to school and spend little or no time outdoors ,the number of short-sighted peoplegets larger. We are also seeing more and more short-sighted children in cities all around the world –and the main reason may be that city children spend less time outdoors . Daylight can be hundreds of times brighter than indoor light . But why does playing outside prevent us from becoming short-sighted ? Scientists believe that natural light has a special chemical（化学物质）which stops the eyeball from growing out of shape and prevents people from becoming short-sighted . So go outdoors .It doesn’t matter if that time is spent having a picnic or playing sports . 根据材料内容选择最佳答案，并将其标号填入题前括号内。 小题1:What can reduce a child's chance of becoming short-sighted according to the passage ?
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