If the sum of $n,2n$ and $3n$ terms of an A.P are $S_1,S_2$ and $S_3$ respectively then,$S_3=$_______

Question

$\begin{array}{1 1}(A) \; 2(S_2-S_1)\\(B)\;2(S_1-S_2)\\(C)\;3(S_2-S_1) \\(D)\;3(S_1-S_2) \end{array}$