A planet of small mass m moves around the Sun of mass M, along an elliptical orbit such that its minimum and maximum distance from the Sun are r and R, respectively. Its period of revolution will be:
(A) 2π6GM(r+R)2
(B) 2π3GM(r+R)3
(C) π2GM(r+R)3
(D) 2πGM(r+R)3