Question: 2018 Winter Final - 5
Let $m \geq 2$ and $n \geq 2$ be integers. What does
$$
{m \choose 2} + {n \choose 2} + m \cdot n
$$
count?
(a)
None of the above.
(b)
The number of ways to choose an ordered pair of 2 elements from a set consisting of $m + n$ elements.
(c)
The number of ways to choose a 2-element subset from a set consisting of $m + n$ elements.
(d)
The number of ways to choose a subset from a set consisting of $m + n$ elements.