Without facts, we can’t form a correct opinion, for we need to have actual knowledge _______ our thinking.
A.which to be based onB.which to base onC.on which to baseD.which to base在线课程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.。