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John________ he could improve his exam results, but he did not have enough time to study.
A、knows how
B、knows that
C、knew how
D、knew that
【正确答案】：C
【题目解析】：考查时态和连词。根据从句可判断为过去时，排除A项和B项。how表示“如何”，符合句意。C项是正确的。句意为:“约翰知道如何提高成绩，就是没有足够的时间学习。”