Let C be the circle with centre `(0, 0)` and radius 3 units. The equation of the locus of the mid points of the chords of the circle C that subtend an angle of `(2pi)/3` at its center is (a) `x^2+y^2=3/2` (b) `x^2+y^2=1` (c) `x^2+y^2=27/4` (d) `x^2+y^2=9/4`