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> A set $X$ with a specified topology $\mathcal T$ on it is called a topological space, we denote it by $(X,\mathcal T)$.
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> A subset $F \subset X$ is called a closed set of $X$ if $X-F$ is open in $X$.
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Give an example of countable set $B$ such that $B$ is not a closed set of $E^2$ ( Euclidean space), then give an example of countable $C$ such that $C$ is a closed set of $E^2$ .

I dont't undertsand this topic, any hints or help so i can understand my lecture note. Thank you