Let $X$ be an infinite set and $T$ a topology on $X$ such that each infinite subset of $X$ is closed. Show that $T$ is the discrete topology.
The idea I have is to take two infinite subsets $A$, $B$. These are closed iff $X\setminus A$ is open and $X \setminus B$ is open. Then I want to see if $X \setminus A$ intersection $X \setminus B$ results in a singleton set. But I have problems in order to form the set $B$ properly to make the intersection described above, a singleton set.
Could you please give some suggestion?