If $S_n$ denotes the sum of $n$ terms of an A.P. whose first term is $'a'$.If the common difference $'d'$ is given by $d=S_n-kS_{n-1}+S_{n-2}$,then the value of $k$ is ______

Question

$\begin{array}{1 1}(A) \; 1\\(B)\;2 \\(C)\;3 \\(D)\;\text{None of the above} \end{array}$