In the mobile below, the two cross beams and the seven supporting strings are all massless. The hanging objects are M_{1}=400 \mathrm{~g}, M_{2}=200 \mathrm{~g}, and M_{4}=500 \mathrm{~g}. What is the value of M_{3} for the system to be in static equilibrium?
Answer Choices
A. 300 \mathrm{~g}
B. 400 \mathrm{~g}
C. 500 \mathrm{~g}
D. 600 \mathrm{~g}
E. 700 \mathrm{~g}
