If Farmer Brown plants no seeds on his farm, he gets no harvest. If he plants 1 bag of seeds, he gets 5 bushels of wheat. If he plants 2 bags, he gets 9 bushels. If he plants 3 bags, he gets 12 bushels. A bag of seeds costs $120, and seeds are his only cost. Farmer Brown’s production function exhibits:

a. increasing marginal product
b. constant marginal product
c. diminishing marginal product
d. the production function is unrelated to the marginal product.