Find $a,b$ and $n$ in the expansion of $(a+b)^n$ if the first three terms of the expansion are $729,7290$ and $30375$ respectively.

Question

$\begin{array}{1 1}(A)\;a=2,b=4,n=7\\(B)\;a=3,b=5,n=6\\(C)\;a=4,b=5,n=8\\(D)\;a=4,b=6,n=10\end{array} $