[pbhjpbhj]

The reason for the question, IMO, is not to get the given answer.

The reason is to learn why there is an answer, at which point you can give an opinion as to whether cm/mm are better. You've missed the primary step, and almost certainly are - as others point out - over qualified for this question (which is a precursor to discussions of decimal precision).

Often when helping the kids with homework I find myself answering "well this is the answer they want ... but the real answer is ...".

tl;dr the question is there to make you think, not because there's only one answer.