A planet moves around the sun. At a given point P, it is closest to the sun at a distance d_1 and has a speed v_1. At another point Q, when it is farthest from the sun at a distance d_2, its speed will:

a. v_2 = v_1.d_1/d_2
b. v_2 = d_1.v_2/d_2
c. v_2 = d_2.v_1/d_1
d. v_2 = v_1.d_2/d_1