In how many ways can we select two disjoint subsets $A$ and $B$ of $\{1,2,\ldots,n\}$? (The internal order in $A$ and $B$ is irrelevant, but it matters which set is $A$ and which is $B$.) . $3^n$ . $2^n$ . $2^n+2^n$ . ${n \choose n/2}$ .