If $\tan\alpha=\large\frac{1}{7}$$,\tan \beta=\large\frac{1}{3}$,then $\cos 2\alpha$ is equal to

Question

$\begin{array}{1 1}(A)\;\sin 2\beta&(B)\;\sin 4\beta\\(C)\;\sin 3\beta&(D)\;\cos 2\beta\end{array} $