The wall shear stress are related by

Question

For steady, fully developed flow inside a straight pipe of diameter D, neglecting gravity effects, the pressure drop $\triangle$p over A length L and the wall shear stress $\tau _{w}$  are related by                                       

      (a) $\tau _{w}=\frac{\triangle pD}{4L}$                                                        (b)$\tau _{w}=\frac{4\triangle pD^{2}}{4L^{2}}$

      (c)$\tau _{w}=\frac{\triangle pD}{2L}$                                                       (d) $\tau _{w}=\frac{4\triangle pL}{D}$