答案：B

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。