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There are two spheres with constant, identical density. The first has a mass of $M$ and a radius of $r$, and the second has a mass of $8M$ and a radius of $2r$.

Which of the following correctly calculates the gravitational force between them if they are touching?

A

$\cfrac { 4G{ M }^{ 2 } }{9 { r }^{ 2 } }$

B

$\cfrac { 8G{ M }^{ 2 } }{9 { r }^{ 2 } }$

C

$\cfrac { G{ M }^{ 2 } }{ { r }^{ 2 } }$

D

$\cfrac { 2G{ M }^{ 2 } }{ { r }^{ 2 } }$

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