QUESTION 13 3 ranchers use a grassland to feed their cows. The ranchers must choose how many cows to raise. The benefit of raising a cow for a rancher is 100- 10 3 G per cow, where ~ is the total quantity of cows using the grassland. The marginal cost of raising a cow is 10. Hence, the net benefit of raising cows for any rancher is (100 - 10 3 G2)g - 10g, where g is the quantity of cows raised by the rancher. What is the social optimum quantity of cows using the grassland? O 3 O 6 O 9 O 12