Let the state of stress at a point in a body be the difference of two plane states of stress shown in the figure. Consider all the possible planes perpendicular to the $x-y$ plane and passing through that point. The magnitude of the maximum compressive stress on any such plane is $k \sigma_{0}$, where $k$ is equal to $\_\_\_\_\_\_\_\_\_\_\_$. (round off to one decimal place).
- $3.0$
- $2.1$
- $1.7$
- $1.5$
