Countable Union/Intersection of Open/Closed sets

A set A is called a F set if it can be written as the countable union of closed sets. A set B is called a G set if it can be written as the countable intersection of open sets.

a) Show that a closed interval [a,b] is a G set

b) Show that the half-open interval (a,b] is both a G and an F set.

c) Show that Q is an F set and the set of irrationals I forms a G set.

a) Let [a,b] be a closed interval.

Well, I know a set is closed if it contains all its limit points

I'm kinda confused on this whole idea.