If $A$ and $B$ are positive acute angle and $\cos A=\large\frac{1}{7},$$\cos B=\large\frac{13}{14}$,then show that $A-B=60^{\large\circ}$

Question

$\begin{array}{1 1}(A)\;\large\frac{1}{2}&(B)\;\large\frac{1}{4}\\(C)\;\large\frac{1}{6}&(D)\;\large\frac{1}{8}\end{array} $