Imagine two planets, A and B, that are made of the same type of material. Planet A has twice the radius of planet B (and so has more material). How would you expect their masses to compare?
    A ) if the radius of planet A is twice as big as B, the mass would be twice as big
    B) if the radius of planet A is twice as big as B, the mass would be 2² = 4 times as big, because the surface area goes as the radius squared (4 π R²)
    C) if the radius of planet A is twice as big as B, the mass would be 2³ = 8 times as big, because the value goes as the radius cubed (4/3 π R³)
    D) they would have the same mass

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