O is the centre of an equilateral triangle ABC $\overrightarrow {F_1} $ , $\overrightarrow {F_2} $ , $\overrightarrow {F_3} $ are three forces acting along the sides AB, BC and AC as shown in fig. What r should be the magnitude of $\overrightarrow {F_3} $ so that total torque about O is zero : :

Question

https://clay6.com/mpaimg/pc40.jpg

( A ) $|\overrightarrow {F_3}|$ = $|\overrightarrow {F_1}|$ + 2 $|\overrightarrow {F_2}|$
( B ) $|\overrightarrow {F_3}|$ = $|\overrightarrow {F_1}|$ + $|\overrightarrow {F_2}|$
( C ) Not possible
( D ) $|\overrightarrow {F_3}|$ = $|\overrightarrow {F_1}|$ - $|\overrightarrow {F_2}|$