问题补充：
Was it through Mary, was working at a high school, you got to know Tom?
A. who; who
B. that; which
C. who; that
D. who; which
答案：
A. who; who
B. that; which
(答案→)C. who; that
D. who; which
解析：C考查定语从句及强调句。句意：你是不是通过正在高中工作的玛丽认识汤姆的？第一空引导非限制性定语从句，修饰先行词Mary，并在从句中作主语，故用who；第二空用that，与Was it through Mary一起构成强调句的一般疑问形式，被强调部分是作方式状语的介词短语through Mary。故C项正确。
