which

pronoun

1. (interrogative) What one or ones (of those mentioned or implied).“By now, you must surely know which is which.”
2. The/Any ones that; whichever.“Please take which you please.”
3. (relative) Introduces a relative clause giving further information about something previously mentioned.“He had to leave, which was very difficult.”
4. (relative) Used of people (now generally who, whom or that).