Two thin rods AB & CD of lengths 2a & 2b move along OX & OY respectively, when 'O' is the origin. The equation of locus of the centre of the circle passing through the extremeties of the two rods is: a. `x^2+y^2=a^2+b^2` b. `x^2-y^2=a^2-b^2` c. `x^2+y^2=a^2-b^2` d. `x^2-y^2=a^2+b^2`