Today, we’ll discuss a number of cases beginners of English fail to use the language prop

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A. which
B. as
C. why
(答案→)D. where
解析：where在此引导定语从句，修饰先行词cases，where的意思相当于in which。类例：There are cases where the word “mighty” is used as an adverb. 在一些情况下，mighty一词可用作副词。