pollute     proper    six     powerful      they

51．Schools should do something to help students protect　　．

52．If a river is　　，farmers can't use the water for their crops．

53．The Internet is more　　 than books．It is influencing our lives a lot．

54．The students who are good at writing can always use words　　．

55．Children usually attend a primary school at the age of　　 in China．