问题补充：
.Was it through Mary , was working at a high school , you get 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：在此句中Was it…that…也的确是强调句型，但句中的两个逗号不在强调句型中，它的作用是把非限制性定语从句who was working at a high school与句子其它部分分隔开来。整句话的汉语意思是：玛丽在一所中学工作，你是不是通过她认识汤姆的？
