If $\large\frac{a^{n+1}+b^{n+1}}{a^n+b^n}$ is the A.M. between $a$ and $b$,then the value of $'n'$ is ______

Question

$\begin{array}{1 1}(A) \; 1\\(B)\;\large\frac{1}{2} \\(C)\;-1 \\(D)\;0 \end{array}$