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Two planets have the same average density but their radii are ${R}_{1}$  and ${R}_{2}$. If acceleration due to gravity on these planets be ${g}_{1}$ and ${g}_{2}$ respectively, then

(a)    $\frac{{g}_{1}}{{g}_{2}}$$\frac{{R}_{1}}{{R}_{2}}$                   (b) $\frac{{g}_{1}}{{g}_{2}}$=$\frac{{R}_{2}}{{R}_{1}}$

(c)    $\frac{{g}_{1}}{{g}_{2}}$$\frac{{{R}_{1}}^{2}}{{{R}_{2}}^{2}}$                   (d) $\frac{{g}_{1}}{{g}_{2}}$$\frac{{{R}_{1}}^{3}}{{{R}_{2}}^{3}}$