Complete the sentences with the given words in their proper forms（用括号中所给单词的适当形式完成下列句子，每空格限填一词)  (共8分)
Saturday is the ___________ day of a week. (seven)
Usually people are afraid of ___________ because they may eat humans. (wolf)
Lei Feng is a good example for us to learn to ___________ the people. (service)
If people still don’t care enough for the environment, the air will become much ___________. (bad)
We can learn a lot from ___________ events. (history)
It is good for you to do all the homework ___________. (you)
Being kind to others and giving a hand to them when necessary is ___________helpful.
(usual)
In order to be safer, we have to ___________ that bridge. (builD.