If $m$ times the $n^{th}$ term of an A.P. is equal to $n$ times the $n^{th}$ term,then its $(m+n)^{th}$ term is _______ ?

Question

$\begin{array}{1 1}(A) \; 0\\(B)\;m\\(C)\;n \\(D)\;m+n\end{array}$