If $A$ and $B$ are positive acute angles and $\cos A=\large\frac{1}{7},$$\cos B=\large\frac{13}{14}$ then find $A-B$

Question

$(a)\;60^{\large\circ }\qquad(b)\;30^{\large \circ}\qquad(c)\;45^{\large\circ}\qquad(d)\;90^{\large\circ}$