根据下列句子的意思及所给单词的首字母或汉语提示，写出所缺单词，每空限填一词。

1．Do you know the Four Great I____________ of Ancient China?

2．In g__________, he can get along well with his classmates．

3．The apartment is so expensive that Mr. Green can’t a___________ it．

4．How to protect ourselves should be ___________ （教） at school．

5．The math problem is too difficult． __________ （无一人） of us can work it out in our class．