Open Set is G-Delta Set

Theorem
Let $T = \struct {S, \tau}$ be a topological space.

Let $U$ be an open set of $T$.

Then $U$ is a $G_\delta$ set of $T$.

Proof
$U$ is the intersection of a singleton.

So $U$ is trivially the intersection of a countable number of open sets of $T$.

The result follows by definition of $G_\delta$ set.