Which of these relations is a one-to-one function?

I. A country to its flag
II. A town or municipality to its zip code
III. A number to its square
IV. The cost of buying a particular brand of ballpen to the number of ballpens bought

A. I and II
B. I and IV
C. I, II, and IV
D. All of these

(please put explanation after the answer)
(let us all be respectful, avoid rudeness)
(please do not answer this if you don't know th right answer)

rude=report
proper answer=brainliest​