1、填which或that,不用where是因为vist后面直接跟地点名词,vist the factory,如果这时候,动词改成work,则应该填where或in which,因为是work in the factory.总结,which或that只能指代名词,这里是factory,而where指代的是in the factory
2、可以.我们知道who是主格,就是说要作主语,得放在动词前面；而whom是宾格,就是说它只能做宾语,得放在动词或者介词的后面.但常常提问时有用who代替whom,例如：Who are you?Who are you waiting for?