The theory he had stuck to be true.A. provedB. to provingC. to proveD. to proved

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@@A. [email protected]@B. to [email protected]@C. to [email protected]@(答案→)D. to proved
解析： 一看这四个选项认为是不定式后接动词原形，其实he had stuck to是充当后置定语，stick to(坚持)为一个固定搭配，去掉这个定语从句后，我们可以很清楚地发现整个句子缺少了谓语动词，定语从句时态为过去完成时，所以主句谓语动词应该过去式。