At a section of a beam, shear force is F with zero BM. The cross-section is square with side a. Point A lies on neutral axis and point B is mid way between neutral axis and top edge, i.e. at distance a/4 above the neutral axis. If $\tau _{a}$ and $\tau _{b}$ denote shear stresses at points A and B, then what is the value of $\frac{\tau _{a}}{\tau _{b}}$?
(a) 0 (b) 3/4
(c) 4/3 (d) None of above