[单选] John________he could improve his exam results, but he did not have enough time to study．

题目：[单选] 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
解析：考时态和连词。根据从句可判断为过去时，排除选项A和B。how表示如何，符合题意。选项C是正确的。【句子大意】约翰知道如何提高考试成绩，就是没有足够的时间学习。