A block is subjected to normal stresses $$\sigma x$$ and $$\sigma y$$ and shear stresses $$\tau$$ along two planes at right angles. This principal Planes inclined $$\theta^\circ$$ to $$\sigma x$$ axis will be such as:

A

$$\tan 2\phi = \frac{2\tau}{\sigma y - \sigma x }$$

B

$$\tan 2\phi = \frac{-2\tau}{\sigma x - \sigma y }$$

C

$$\tan 2\phi = \frac{-2\tau}{\sigma y - \sigma x }$$

D

None of these