ABC is an equilateral triangle with O as its centre. $\overrightarrow{{\mathrm{F}}_1}, \overrightarrow{{\mathrm{F}}_2} \mathrm{and} \overrightarrow{{\mathrm{F}}_3}$ represent three forces acting along the sides AB, BC and AC respectively. If the total torque about O is zero then the magnitude of $\overrightarrow{{\mathrm{F}}_3}$ is 

• A

  $2({\mathrm{F}}_1+{\mathrm{F}}_2)$

• B

  ${\mathrm{F}}_1 + {\mathrm{F}}_2$

• C

  ${\mathrm{F}}_1 - {\mathrm{F}}_2$

• D

  $\frac{{\mathrm{F}}_1+{\mathrm{F}}_2}{2}$