Without facts,we cannot form a worthwhile opinion,for we need to have factual knowledge our thinking.
A. upon which to base
B. which to be based on
C. which to base on
D. to which to be based on
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(答案→)A. upon which to base
B. which to be based on
C. which to base on
D. to which to be based on
解析：全句意思为：没有事实，我们就不能形成有价值的观点，因为我们的思考必须以真实的知识为基础。故选A。upon which to base our thinking是动词不定式短语作定语，该短语中介词必须放在which前。参见必背句型2。
答案：A