If the coefficients of $x^2$ and $x^3$ in the expansion of $(3+kx)^9$ are equal then the value of $k$ is

Question

$\begin{array}{1 1}(A)\;-\large\frac{9}{7}\\(B)\;\large\frac{9}{7}\\(C)\;\large\frac{7}{9}\\(D)\;\text{None of these}\end{array} $