1. He is the only student______ understands English well.
A. who         B. that        C. who / that

答案选 C。因先行词有 the only 修饰，通常用 that 不用 which 来引导定语从句。但若先行词指人，则用 that, who 均可。此用法用 that 没有问题，估计有的同学认为不能用 who，下面是用 who 的词典例句：

I was the only one who disagreed. 我是唯中不同意的人。（朗文）

I put it to you that you are the only person who had a motive for the crime. 我让你明白你是唯一具有犯罪动机的人。（牛津）

2. That is the horse______ won the race.
A.which       B.that         C. which / that
答案选 C。先行词指事物或动词，其后的定语从句用 that, which 引导定语从句均可。