three dimensional state of stress point

Question

The three-dimensional state of stress at a point is given by:

$\left [ \sigma \right ]=\begin{bmatrix} 30 &10 &-10 \\ 10&0 &20 \\ -10& 20 &0 \end{bmatrix}MN/m^2$

The shear stress in the x-y plane at the same point is then equal to
(a) zero MN/ $m^2$                       (b)–10 MN/$m^2$
(c) 10 MN/$m^2$                           (d) 20 MN/$m^2$