ABC is an equilateral triangle with O as its centre $\overrightarrow{F_1},\overrightarrow{F_2} and \overrightarrow{F_3}$ represent three forces acting along the sides AB, BC and AC respectively. If the torque about O is zero then the magnitude of $\overrightarrow{F_3}$ is

Question

( A ) $2 (F_1 + F_2)$
( B ) $F_1 + F_2$
( C ) $F_1 – F_2$
( D ) $\frac{F_1+ F_ 2}{2}$