I can think of many cases students obviously know a lot of English words and expressions b

问题补充：

I can think of many cases students obviously know a lot of English words and expressions but couldn’t write a good essay .

A. why

B. which

C. as

D. where

答案：

A. why

B. which

C. as

(答案→)D. where

解析：考查定语从句。本题定语从句的先行词是many cases，定语从句的句子结构很完整，故从句中使用关系副词.句意：我能想起很多情况，在这些情况下，学生知道很多词汇和短语，但是却写不出好的文章。根据句意说明是指在这里情况之下，故使用where。D正确。