Two stars of masses $m_1$ and $m_2$ are parts of a binary star system. The radii of their orbits are $r_1$ and $r_2$ respectively,measured from the centre of mass of the system. The magnitude of the gravitational force that $m_1$ exerts on $m_2$ is

• A
  $\frac{G m_1 m_2}{(r_1 + r_2)^2}$
• B
  $\frac{G m_1}{(r_1 + r_2)^2}$
• C
  $\frac{G m_2}{(r_1 + r_2)^2}$
• D
  $\frac{G(m_1 + m_2)}{(r_1 + r_2)^2}$