Spending two or three hours playing outdoors each day can reduce the chance of becoming short-sighted, a research shows. It challenges the belief  that short-sightedness is caused by computer use, watching TV or reading in weak light. The Australian government researchers believe that the sunlight is good for people's eyes. They compared the vision  and habits of 100 seven-year-old children in Singapore and Australia. In all, 30% of the Singaporean children were short-sighted--this rate  was ten times higher than Australian children. Both groups spend a similar amount  of time reading, watching television and playing computer games. However the Singaporean children spent an average   of only half an hour a day outdoors--90 minutes less than the Australian children. Professor Ian Morgan, from the Australian Research Council's vision Centre, 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 people gets larger. We're 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  which stops the eyeball from growing out of shape and prevents people from becoming short-sighted. So be outdoors. It doesn't matter if that time is spent having a picnic or having sports. How much time did the Australian children spend outdoors on average according to the research?
30 minutes. 60 minutes. 90 minutes. 120 minutes.