The number of 3-element subsets of an $(n-1)$-element set.

The number of 3-element subsets of an $n$-element set.

The number of times you fart when running algorithm $\mathrm{S}\mathrm{I}\mathrm{L}\mathrm{L}\mathrm{Y}(n)$.

The number of 3-element subsets of an $(n+1)$-element set.