Without facts,we can't form a worthwhile opinion for we need

Without facts,we can't form a worthwhile opinion for we need to have factual knowledge( )our(见下方
thinking
A.which to be based on B.which to base upon C.upon which to base
D.to which to be based
该题正确答案应选C,请解释一下原因,说明其他几项为什么不对,谢谢