is surprising to us is that Tom, for English was once boring, can speak English fluently

is surprising to us is that Tom, for English was once boring, can speak English fluently as though he were a native speaker.
A. What; whom
B. What; whose
C. It; whose
D. As; whom

(答案→)A. What; whomB. What; whoseC. It; whoseD. As; whom解析：考查主语从句和定语从句：句意：让我们惊讶的是汤姆对他来说英语曾经是枯燥的，现在可以流利的说英语好像是英国本土的人。通过分析句子结构，可知这个句子中包含一个主语从句，和一个非限制性定语从句。且引导词在主语从句中作主语，表示“什么”的含义，故用what。介词 + 关系代词引导的定语从句，定语从句的主语是English，需要for的宾语，指人只能用whom。选A。

感谢回答