A

In China，more and more middle school students are getting shorter sleeping time than be-fore． Most students sleep less than nine hours every night，because they have much homework to do. Some homework is given by their teachers，and some by their parents． Also，some students don't know how to save time .They are not careful enough while they do their homework，so it takes them a Iot of time． Some students spend too much time watching TV or playing computer games．They stay up very late． Some students have to get up early every morning on weekdays to get to school on time by bus or by bike．lt can be a long way from home to school．

Schools and parents should cut down some of the homework so that our children can enjoy more than nine hours of sleep every night for their health． For children，we should make best use of our time． When we have enough time for sleeping，we will find it much better for both our study and health．

( )21． What makes most students sleep less than nine hours every night?

A． Watching TV．

B． Playing computer games．

C． Doing their homework．

D． Doing housework．