Question: 2015 Winter Midterm - 6
$\newcommand{\Silly}{{\rm S \scriptsize ILLY}}$
What does
$$
\sum_{k=2}^{n-1} (k-1)(n-k)
$$
count?
(a)
The number of 3-element subsets of an $(n+1)$-element set.
(b)
The number of 3-element subsets of an $(n-1)$-element set.
(c)
The number of 3-element subsets of an $n$-element set.
(d)
The number of times you fart when running algorithm $\Silly(n)$.