When the values of gravitational force F are plotted on the y-axis and the corresponding values of r21 are plotted on the x-axis for a fixed pair of masses, which of the following best describes the expected graph?
a) A curve that becomes less steep but never touches either axis
b) A straight line with negative slope that passes through the origin
c) A straight line passing through the origin
d) A straight line that intersects the y-axis above the origin