A circle of constant radius 'a' passes through origin 'O' and cuts the coordinate axes in points P and Q, then the equation of the locus of the foot of perpendicular from O to PQ is
(a) (x^2+y^2)(1/x^2+1/y^2)=4a^2
(b) (x^2+y^2)(1/x^2+1/y^2)=a^2
(c) (x^2+y^2)^2(1/x^2+1/y^2)=4a^2
(d) (x^2+y^2)(1/x^2+1/y^2)=a^2