The point `P` moves in the plane of a regular hexagon such that the sum of the squares of Its distances from the vertices of the hexagon is `6a^2`. If the radius of the circumcircle of the hexagon is `r(< a)`, then the locus of P is : a) A pair of straight lines b)An ellipse c)A circle of radius `sqrt(a^2-r^2)` d)An ellipse of major axis `a` and minor axis `r`.