Gravitational Force

The gravitational force between two objects is described by the following formula.
$F = G\cfrac{m_1 \times m_2}{d^2}$ where $F$ is the gravitational force, $G$ is the gravitational constant, $m_1$ and $m_2$ are the masses of the two objects, and $d$ is the distance between the items.
Assume that the two objects are $15$ meters apart at first. They are then moved $30$ meters apart. How much is the gravitational force reduced?
A. $\cfrac{1}{4}$
B. $\cfrac{1}{6}$
C. $\cfrac{1}{2}$
D. $\cfrac{1}{3}$
E. $\cfrac{1}{8}$
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