In Newton's law of universal gravitation, how does the gravitational force depend on the sizes (masses) of two objects separated by a distance $r$?

• A. It depends only on distance and not on mass: $F=G\frac{1}{r^2}$.
• B. It is inversely proportional to the product of their masses: $F\propto \frac{1}{m_1\times m_2}$.
• C. It is directly proportional to the product of their masses: $F=G\frac{m{m}_1\times m_2}{r^2}$.
• D. It is directly proportional to the sum of their masses: $F\propto m_1+m_2$.

Answer: (C)