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

问题补充：

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; whose

B. What; whom

C. It; whose

D. As; whom

答案：

A. What; whose

(答案→)B. What; whom

C. It; whose

D. As; whom

解析：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，指人只能用whom。