答案：C

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。