How Did Newton Deduce the Dependence of the Gravitational Force

Multiple Choice

How did Newton deduce the dependence of the gravitational force on distance?

A) By measuring the gravitational attraction between large massive lead spheres in the laboratory.
B) By observing the way that a large gravitational pendulum moved.
C) By comparing the accelerations of objects falling to Earth from different heights.
D) By comparing the time it takes Earth to orbit the sun with the time it takes to spin on its axis.
E) By calculating the moon's acceleration, and comparing it with the acceleration of a falling object.