15.The professor could hardly find necessary grounds_______his arguments in favour of the new theory.
A.to be based on B.to base on
C.which to base on D.on which to base
解析：句意：这位教授几乎不可能找到必需的理由，以此来支持新学说。
本题考查定语从句和base 一词的用法。固定短语：base sth on sth。
原句剖析：The professor could hardly find necessary grounds（ on which to base his arguments）in favour of the new theory.划线部分为定语从句，正常语序应该是which he base his arguments on介词提前可以得到on which he base his arguments，大家都知道宾语从句中whatto do=whatwe shoulddo ，类比可得，此句可改成on which to base his arguments
答案：D