单选题

　　[阅读选择]Today， working less than a 40-hour week， people enjoy more leisure time. Hence， the wise use of leisure time has become an important problem for everyone， young or old. It is a particularly difficult problem for the sick， the aged， and those who have retired from earning a living. Those people have so much leisure that it is hard for them to find interesting and worthwhile ways to use it.

　　How to spend leisure time wisely is not a particularly difficult problem for people ______.

　　A.who are very busy everyday

　　B.who are sick in bed

　　C.who are aged and in good health

　　D.who have retired from work