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 $r_{a}=2 R$ and $r_{b}=R$ , respectively. What is the ratio of their periods $\mathrm{T}_{\mathrm{a}} / \mathrm{T}_{\mathrm{b}}$ ?

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