B. who
当关系代词指人时，关系代词既可用who，也可以用that。但在以下情况用who。
1.当先行词是persons, people, those时。例如：
Those who are for my plan , please put up your hands.同意我的计划的人请举手。
Persons who are quarrelsome are despised.好争者被轻视。
2.当先行词是he, one, all, any, they等时，例如：
He who has not reached the Great Wall isn't a true man. 不到长城非好汉。
All who had seen this film were moved.看过这部电影的人都受感动。
3.在非限定性定语从句中用who。例如：
I want him, who knows some English and French. 我要他，他懂得些英语和法语。
I will pardon you, who are honest. 我愿意宽恕你，你是诚实的。
4.在强调句型中多用who，在口语中who可以省略。例如：
I think it is you who should come here tomorrow.我认为你明天应该来这儿。
It is he who can do this work. 只有他能够干此工作。
在以下情况多用that。例如：
5.当关系代词在从句中作表语时用that。例如：
She is not the woman that she used to be.她已经不是过去的她了。
China is not the country that it used to be.中国已经不是过去的中国了。
6.避免重复。例如：
Who was it the man that you danced with last night? 昨天晚上和你一起跳舞的那个人是谁？
Who is it the girl that often comes to school early? 常常早到校的那个女孩是谁？
7.当先行词前面有指示代词same时，要用that。例如： That is the same woman that spoke at the meeting yesterday.这个与昨天在会议上讲话的是同一个人。
He is the same person that killed that woman.他就是杀害那位妇女的人。
8.当先行词既有人，又有物时。例如：
He was watching the children and parcels that filled the car.他望着塞满车的孩子和包裹。
Let’s talk about the persons and the things that we can remember.让我们谈论能够想起的人和事。
9.当先行词前面有形容词最高级、序数词或only等词修饰时。例如：
I knew her father for the simplest, hardest working man that ever drew the breath of life. 我早知她的父亲是一个世上最简朴最努力工作的人。
参考资料：http://hi.baidu.com/ellamoon/blog/item/41c9be514c9a5c2042a75b09.html