In the expansion of $(1 + a)^{m+n}$, prove that coefficients of $a^m$ and $a^n$ are equal.

Question

$\begin{array}{1 1}\text{They are equal} \\ \text{They are both 0} \\ a^m \text{is greater than} a^n \\ a^m \text{is lesser than} a^n\end{array} $