这个句子是主系表结构 The school（___Mr.smith teaches ）is a world-famous one. 括号里的是定语从句,修饰The school的,恢复这个定语从句应该是Mr.smith teaches in The school 如果把The school提到前面去,那就应该再加一个in,因此这个空应该填in which,而定语从句的先行词中in which=where,两者完全等价
The school WHERE Mr.smith teaches is a world-famous one. ==The school IN WHICH Mr.smith teaches is a world-famous one. === The school IN WHICH (==in the school ) Mr.smith teaches is a world-famous one.
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