During the measurement of viscosity of air flowing through a pipe, we use the relation $\mu = \frac{\pi d^{4}}{128Q}(-\frac{dp}{dx})$ under the condition that in the measuring section
(a) There is a viscous zone near the wall and an in viscid core persists at the centre
(b) The entire cross- section is viscous
(c) The flow can be assumed as potential flow
(d) The flows is irrotational