Without facts, we can’t form a correct opinion, for we need to have actual knowledge _______ our thinking.

A.which to be based on

B.which to base on

C.on which to base

D.which to base

答案及解析： C

解析：

此空的不定式作定语修饰其前的名词，构成have sth. to do结构，不定式用主动表示被动；

如果用“疑问句+不定式”作定语，则不定式中的介词要放在疑问词前；

D项缺少介词on。该句也可写成：...for we need actual knowledge to base our thinking on.。