there are many things in different areas__we can do to reduce the possible danger of AIDS. A that

B which为什么不选B?

that后加从句修饰areas
定语从句的关系代词 Which的用法: 若先行词指物则代替先行词在定语从句中充当主语或宾语.

This is the pen which was given by my friend.

先行词 指物代替先行词在定语从句中充当主语

This is the pen which my friend gave to me.

先行词 指物代替先行词在定语从句中充当宾语

下面是他和that在定语从句中的区别及用法：
that \which在代物时常常可以通用,但有时只宜于用which ,不用that

(1) 关系代词前有介词时.

This is the hotel in which you will stay.

(2) 如有两个定语从句,其中一句的关系代词是that,另一句宜于用which.

Let me show you the novel that I borrowed from the liberary which was newly open (新开放) to us.

注意2that \which在代物时常常可以通用,但有时只宜于用that ,不用which.

(1) 先行词是形容词最高级或者它的前面有形容词最高级时

This is the best that has been used against air pollution(反对空气污染) in cities.

English is the most difficult subject that you will learn during these years.

(2) 先行词是序数词,或它的前面有一个序数词时

He is the last person that I want to see.

(3) 主句中已有疑问词时

Which is the bike that you lost?

(4) 先行词既有人又有物时

The bike and its rider that had run over (撞倒了)an old man were taken to the police station.

(5) 先行词是all,much,little,something,everything,anything,nothing,none,the one等代词时

You should hand in all that you have.

We haven’t got much that we can offer you.

I mean the one that you talked about just now.

(6) 先行词前面有only,any,few,little,no,all,one of,the same,the very等词修饰时

The only thing that we can do is to give you some money.

Li Ming is the only one that got full marks (满分)in our class.

Li Ming is one of the students that want to be teachers in our class.

(7) 有两个定语从句,其中一个关系代词已用which,另外一个宜用that

Edison built up a factory(办了一个工厂) which produced things that had never been seen before.

应该是因为先行词“ many things”是复数的原因吧

先行词是不定代词时 ，l例如something nothing anything等，定语从句的引导词只能用that，因为这个句子的先行词是不定代词many things