Hi all

I have the following table with 3 columns

car_id | car_reservationuser_id | comments
----------------------------------------------------
A | 12 | requre car on sep 12
B | 13 | on sep 13
A | 13 | on sep 14
C | 17 | on oct 2
B | 17 | on oct 12
A | 17 | on sep 27


My question is ...
To which column do you place the primary key in this table and why?

This question i have been asked in a interview.

Thanks in advance