Which beginning (prefix) or ending (suffix) can the following lists of words have in...

Question

Which beginning (prefix) or ending (suffix) can the following lists of words have in common?
E.g. risk – wind – health: y

Dress – do – happy – lucky

Answer 1

Preffix un.
undress. undo. unhappy. unlucky. 

Answer 2

Dress – do – happy – lucky -
undress - undo-unhappy-unlucky