In an experiment to determine the rheological behaviour of a material, the observed relation between shear stress, $\tau$, and rate of shear strain, $\frac{du}{dy}$, is $\tau = \tau _{0} + C(\frac{du}{dy})^{0.5}$, The material is
(A) A Newtonian fluid
(B) A thixotropic substance
(C) A Bingham plastic
(D) An ideal plastic