7. A piece of thin uniform wire of mass m and length 3b is bent into an equilateral triangle. Find the moment of inertia of the wire triangle about an axis perpendicular to the plane of the triangle and passing through one of its vertices.
(A)2mb2/3
(B) 7mb2/4
(C) 1mb2/3
(D) 1mb2/4
(E) 1mb2/2.
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