Three point masses $m_1, m_2, m_3$ are placed at the vertices of an equilateral triangle of side $a$. The moment of inertia of the system about an axis passing through the vertex $m_1$ and perpendicular to the plane of the triangle is not what is asked; rather,the axis passes through $m_1$ and bisects the opposite side $m_2m_3$. What is the moment of inertia of the system about this axis?

• A
  $ (m_2 + m_3) \frac{a^2}{4} $
• B
  $ (m_1 + m_2 + m_3) a^2 $
• C
  $ (m_1 + m_2) \frac{a^2}{4} $
• D
  $ (m_2 + m_3) a^2 $