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 total-cost curve is: __________

a. increasing at a constant rate.
b. increasing at an increasing rate.
c. increasing at a decreasing rate.
d. decreasing.