this is the only person --i know in beijing . A who B that

1。 这种题目没有太大意义。
2。 按照定语从句的语法来说：当先行词词被the very, the only,the same,the last修饰时,关系代词最好用that。 如：
1) This is the very good dictionary that I want to buy。
2) After the fire in his house, the old car is the only thing that he owned。
3。 但是当先行词指人时,偶尔也可以用who。
如：Wang Hua is the only person in our school who will attend the meeting。
4。 the only person做先行词时,还是看the only,而且关系代词的选择跟其在定语从句中做主语还是做宾语没有关系。