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有這么一道題：

The professor can hardly find sufficient grounds ______ his argument in favour of the new theory.  

 A. on which to base  B. which to be based on   C. which to base on

答案為：A

問：請幫忙排除B和C的答案，尤其是C，有的說是定語從句，若是，那么on放在后面為什么不行呢？

謝謝

thanks a lot.