Three particles,each of mass $m$,are placed at the vertices of an equilateral triangle $ABC$ of side length $l$. What is the moment of inertia of the system about the axis $AX$ in the plane of $ABC$,as shown in the figure?

• A
  $\frac{3}{4}ml^2$
• B
  $2ml^2$
• C
  $\frac{5}{4}ml^2$
• D
  $\frac{3}{2}ml^2$