A block is subjected to normal stresses σ x and σ y and shear stresses τ along two planes at right angles. This principal Planes inclined θ° to σ 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