maximum shear stress

Question

In a plane stress condition, the components of stress at a point are $\sigma _x$ = 20 MPa, $\sigma_y$ = 80  MPa and  $\tau _{xy}$ = 40 MPa. The maximum shear stress (in MPa) at the point is

 (a) 20           (b) 25          (c) 50            (d) 100