was wondering if someone could shine some light on the highlighted half of this question? Any help would be greatly appreciated. Please excuse me for the poor format of the question, I'm new to this! Thanks in advance.
Determine whether the following set is closed: $$D = \{z\text{ is complex}: \Re(z)\ge\frac13 \text{ or } \Re(z)\le2\}$$ Is the set $D$ open? Justify your answer.
Hint
$D=\mathbb C$, so the question is:
Is $\mathbb C$ open?