meaning of "Paper back"

[pupil]

Test No. incompl/advan-50 "Papers", question 10

Her first novel was a huge success and it was only a few weeks after publication that the paper ......... came out.

(a) side
(b) edition
(c) back
(d) book

Test No. incompl/advan-50 "Papers", answer 10

Her first novel was a huge success and it was only a few weeks after publication that the paper back came out.

Correct answer: (c) back

Hello Alan! I didn't find out it! ...Paper back...
Explain please! Thank you
Pupil

[Alan]

Hi,

A book is usually published as a hardback first and then it is sold in a cheaper version as a paperback, which is the sort available in airports for example.

Alan
_________________
English as a Second Language
You can read my ESL story Saying It Twice

[pupil]