—Where did you pick up the stamp?—It was at the post office we worked in the summer vacat

问题补充：

—Where did you pick up the stamp?

—It was at the post office we worked in the summer vacation.

A. that

B. where

C. which

D. when

答案：

A. that

(答案→)B. where

C. which

D. when

解析：B 考察定语从句和强调句型。Where引导定语从句 ，在从句做地点状语，修饰先行词the post office，这是强调句的省略形式，句子补充完整是It was at the past office where we worked in the summer vacation that I picked up the stamp。强调的是整个定语从句部分。