Two Planets Travel in Circular Orbits About a Star at Radii

Multiple Choice

Two planets travel in circular orbits about a star at radii of r_a = 2R and r_b = R, respectively. What is the ratio of their periods T_a/T_b?

A) 1.6
B) 2.8
C) 1.3
D) 1.4
E) 2.0