题目内容：

用方框中所给单词的适当形式填空，每题仅用一次，每空限填一词。

pollute     proper    six     powerful      they

1.Schools should do something to help students protect _______ .

2.If a river is _____ , farmers can’t use the water for their crops.

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

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

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