Two planets move around the sun. The periodic times and the mean radii of the orbits are \[{{T}_{1}},\,{{T}_{2}}\] and \[{{r}_{1}},\,{{r}_{2}}\] respectively. The ratio \[{{T}_{1}}/{{T}_{2}}\] is equal to [CPMT 1978]

• A.             \[{{({{r}_{1}}/{{r}_{2}})}^{1/2}}\]
• B.               \[{{r}_{1}}/{{r}_{2}}\]
• C.             \[{{({{r}_{1}}/{{r}_{2}})}^{2}}\]
• D.               \[{{({{r}_{1}}/{{r}_{2}})}^{3/2}}\]