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。
What can reduce a child's chance of becoming short-sighted according to the passage ?

A．Playing computer games.

B．watching TV.

C．Reading books.

D．Spending 2 or 3 hours playing outside every day.

What does the underlined word "percentage" mean?    

A．百分比

B．功率

C．平均分

D．效率

How much time did the Chinese children spend outdoors according to the research?                                   

A．2 hours.

B．90 minutes.

C．1 hour.

D．30 minutes.

What is the fifth paragraph mainly about?

A．Why people become short-sighted.

B．Why natural light has a special chemical.

C． Why playing outside is good for one's eyesight.

D． Why daylight is much brighter than indoor light.

Which of the following is TRUE according to the passage?  

A．People will all become short-sighted after they begin to go to school.

B．Playing outdoors for 2 or 3 hours every day can help protect your eyesight.

C． Children in Australia are more likely to become short-sighted than those in China.

D． lf you spend 2 or 3 hours playing outside each day, you will become short-sighted.