Three Hundred Paper Mills Compete in the Paper Market

Three hundred paper mills compete in the paper market. The total cost of production (in dollars) for each mill is given by the formula , where Q^mill indicates the mills annual production in thousands of tons. The marginal external cost of a mill's production (in dollars) is given by the formula . Finally, annual market demand (in thousands of tons) is given by the formula. . Find the efficient quantity that involves no externalities

A) 34,286
B) 60,000
C) 1,200
D) 90,000