定语从句中先行词是all people时,关系词是用who还是that?

只能用that,见如下第一点：
限制性定语从句只能用that 的几种情况
1．当先行词是anything,everything,nothing (something 除外),few,all,none,little,some 等代词时,或者是由every,any,all,some,no,little,few,much 等修饰时
(1) Have you taken down everything that Mr.Li has said?
(2) There seems to be nothing that seems impossible for him in the world.
(3) All that can be done has been done.
(4) There is little that I can do for you.
注意：当先行词指人时,偶尔也可以用who
(4) Any man that/.who has a sense of duty won't do such a thing.
2.当先行词被序数词修饰
(1) The first place that they visited in London was the Big Ben.
3.当先行词被形容词最高级修饰时
(1) This is the best film that I have seen.
4.当形容词被the very,the only修饰时
(1) This is the very dictionary that I want to buy,
(2) After the fire in his house,the old car is the only thing that he owned.
当先行词指人时,偶尔也可以用who
(3) Wang Hua is the only person in our school who will attend the meeting/
5.当先行词前面有who,which等疑问代词时
(1) Who is the man that is standing there?
(2) Which is the T-shirt that fits me most?
6.当先行词既有人,也有动物或者物体时
(1) Can you remember the scientist and his theory that we have learned?

用that 这只能说是语法书里的概念了 ： 当先行词有the very，the only ,the same ,any,few,little,no,all,one of 等次修饰时应用。
你说的（3）和（4）也是对的 但是当这些先行词前有上述所说词修饰时就改用that了。
LiMing is the only one that wants to be a teacher 懂了。 你说的先行词是people 而people前又用all ，所以要用that了