21．---Where did you come across our chemistry teacher ?
--- It was in the supermarket ______I purchased mooncakes．
A．that B．which C．where D．when
答案为: C. 这是一个省略了的强调句。完整句子是：It was in the supermarket that I purchased mooncakes that I came across our chemistry teacher. 要填的词是强调部分所带定语从句的引导词。
