In a material under a state of plane strain, a $10 \times 10$ mm square centered at a point gets deformed as shown in the figure.

If the shear strain $\gamma_{xy}$ at this point is expressed as $0.001k$ (in rad), the value of $k$ is

- $0.50$
- $0.25$
- $- 0.25$
- $- 0.50$