If a variable circle S touches S1 : |z – z1| = r1 internally and S2 : |z – z2| = r2 externally while the curves S1 & S2 touch internally to each other. Then the locus of the centre of the curve S is
(A) a conic whose centre is (z1 + z2)/2
(B) a conic which does not have centre
(C) a conic whose eccentricity is (r1 - r2)/(r1 + r2)
(D) a conic whose eccentricity is (r1 + r2)/(r1 - r2)
