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：但是许多学生刚好首先排除了C项，他们认为:（1)非限制性定语从句不用关系代词that引导;（2)
强调句型It be + 被强调部分 + that (who)…中，who (that)前不能有逗号。
上述两点是对的，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？
