Which one of the following correctly represents the shear stress distribution across a section of a circular pipe having a viscous flow?
(a) $\tau =\frac{\partial p r^{2}}{\partial X}$
(b) $\tau =\frac{\partial p\left ( r/2 \right )}{\partial X}$
(c) $\tau =-\frac{\partial p\left ( r/2 \right )}{\partial X}$
(d) $\tau =-\frac{\partial p\left ( r \right )}{\partial X}$