Last year Martha could grow 20 bushels of basil or 25 bushels of cilantro on each acre of land that she farmed. This spring, she discovered a free organic fertilizer that allows her to double the amount of cilantro that she can grow on each acre, ceteris paribus. How has Martha's opportunity cost changed with this discovery? 

a. The opportunity cost of cilantro stays the same because the discovery will allow Martha to produce more of both goods.

b. The opportunity cost of cilantro stays the same because the discovery does not change the amount of basil that Martha can produce.

c. The opportunity cost of cilantro has increased.

d. The opportunity cost of cilantro has decreased.