What Does R Stand for in the Following Equation $$F = \frac { G m _ { 1 } m _ { 2 } } { r ^ { 2 } }$$

Multiple Choice

What does r stand for in the following equation? $$F = \frac { G m _ { 1 } m _ { 2 } } { r ^ { 2 } }$$

A) Radius of the earth
B) Universal gravitational constant
C) Resistance
D) Center-to-center distance between objects