用括号内所给动词的正确形式填空.（请注意时态和语态两个方面噢）
1.What______your computer_____(use)for?
2.A new airport____(build)in 1992.
3.What____(must do)before you join the charity walk?
4.It's dark.The seats____(can not find)easily
5.Now skills and knowledge_____(teach)to these workers last time
6.Wild animals____(protect)by people in China