三道简单的英语问题请教高手,谢谢~

三道简单的英语问题请教高手,谢谢~
1、_________the fish farm, we can do the fishing.
请问at/on填哪一个正确?为什么?
2、they are cleaning the classroom. don't walk________ the classroom.
请问为何用into不用in ?
3、workers are sitting on the chairs.(对workers进行提问）
请问为何不能用who are 进行提问而用who's?who's 难道不是对单数提问吗?
烦请赐教!非常感谢!