7. Seven rings are arranged in hexagonal, planar pattern so as to touch each neighbor, as shown in the figure. Each ring is a uniform loop of mass m and radius r. What is the moment of inertia of the system of seven rings about an axis that passes through the center of the center ring and is normal to the plane of the system? (A) 7mr² (B) 11mr² (C) 13mr² (D) 23mr² (E) 31mr²