单选题

　　【阅读理解】Two United Nations agencies warned on Tuesday that children are most at risk of developing skin cancers as a result of the long-term decline in the earth's protective ozone layer. The agencies, the World Health Organization（WHO）and the UN Environmental Programme（UNEP）, issued the warning as they launched a global programme aimed at alerting schools to the dangers of exposure to the sun.

Why does the risk of developing skin cancers in children become greater and greater

　　A、Because they pay little attention to their skin health.

　　B、Because they are short of physical training.

　　C、Because the earth's protective ozone layer declines year after year.

　　D、Because the earth is getting warmer and warmer.