There are many good stories about the Long March, this is the most instructive.
A. of which
B. in which
C. from which
D. by which
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(答案→)A. of which
B. in which
C. from which
D. by which
解析：考察定语从句。本题定语从句的先行词是many good stories,后面的定语从句中句子结构很完整，故使用关系副词where，在定语从句中关系副词=介词+关系代词。本句是指在这些故事中，这是最有教育意义的。故使用of表示所属关系。故A正确。