Give an example of a closed subset of ($\mathbb{C},d$) Give an example of a closed subset of ($\mathbb{C},d$) where $d$ is the standard metric $d(z,w)=|z-w|$ for $z,w \in \mathbb{C}$
I know that a a subset is closed if $(\mathbb{C},d)$ without the subset is open

Okay, then using what you know, what would be an example of an open set $U$? Then $\Bbb C\smallsetminus U$ is closed.
