At certain point in a structural member , there are perpendicular stresses 80 $$\frac{N}{mm^{2}}$$ and 20 $$\frac{N}{mm^{2}}$$,both tensile .what is the equivalent stress in simple tension ,according to the maximum principal strain theory ?(poisson's ratio=0.25)

A

Zero

B

20 $$\frac{N}{mm^{2}}$$

C

60 $$\frac{N}{mm^{2}}$$

D

75 $$\frac{N}{mm^{2}}$$