ABC is an equilateral triangle with `O` as its centre. `F_1, F_2 and 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 `F_3` is
Options:
(a) `F_1+F_2`
(b) `F_1-F_2`
(c) `(F_1+F_2)/2`
(d) `2(F_1+F_2)`