A. who / that / which
B. who = for people (not things)
- A thief is a person who steals things.
- Do you know anybody who can play the piano?
- The man who phoned didn't give his name.
- The people who work in the office are very friendly.
C. that = for things or people
- An aeroplane is a machine that flies.
- Emma lives in a house that is 500 years old.
- The people that work in the office are very friendly.
You can use that for people, but who is more usual.
D. which = for things (not people)
- An aeroplane is a machine which flies. (not 'a machine who ...')
- Emma lives in a house which is 500 years old.
Do not use which for people:
• Do you remember the woman who was playing the piano at the party?
(not 'the woman which ...')
who and which in questions → UNITS 44, 46 the people we met (relative clauses 2) → UNIT 114
Examples
I met a woman who can speak six languages.
Jim was wearing a hat which was too big for him.
A thief is a person who steals things.
An aeroplane is a machine that flies.
The people who work in the office are very friendly.
Unit 113: a person who … / a thing that/which … • Full Interactive Textbook