When a shaft is subjected to combined twisting moment (T) and bending moment (M), the equivalent twisting moment is equal to

A

$$\frac{1}{2}\left[ M +\sqrt{M^2 + T^2}\right]$$

B

$$\sqrt{M^2 + 4T^2}$$

C

$$\sqrt{4M^2 + T^2}$$

D

$$\sqrt{M^2 + T^2}$$