—Where did you come across our Chinese teacher?—It was in the supermarket I purchased moo

问题补充：

—Where did you come across our Chinese teacher?

—It was in the supermarket I purchased mooncakes.

A. that

B. which

C. where

D. when

答案：

A. that

B. which

(答案→)C. where

D. when

解析：C 考察定语从句和强调句型。本句的答句部分是强调句的省略形式。补充完整是：It was in the supermarket I purchased mooncakes that I came across our Chinese teacher。被强调部分包括一个定语从句，先行词是supermarket，在定语从句中做地点状语。故C正确。