Two planets move around the sun. The periodic times and the mean radii of the orbits are

Two planets move around the sun. The periodic times and the mean radii of the orbits are  T₁ , T₂and r_{1 ,}r₂ respectively. The ratio T₁/T₂ is equal to

(a)   ( r_{1 /}r₂ )^1/2                      (b) r_{1 /}r₂

(c)     ( r_{1 /}r₂ )²                      (d) ( r_{1 /}r₂ )^3/2

Answer: d