3. A rod of negligible mass is pivoted at a point that is off-center, so that length l1 is different from length l2. The figures above show two cases in which masses are suspended from the ends of the rod. In each case the unknown mass m is balanced by a known mass, M1 or M2, so that the rod remains horizontal. What is the value of m in terms of the known masses?
(A) M1+M2
(B) 1/2(M1+M2)
(C) M1M2
(D) M1M2