Wu Ti's government bought excess grain from Chinese farmers when it was available and stored it in granaries to keep the price of grain stable in case of a poor growing season. Which of the following statements describes why Chinese farmers were able and willing to sell the excess grain to the government?
A.they were willing to sell excess grain when the supply of grain was greater than the demand for grain
B.they were willing to sell excess grain when the opportunity cost of producing more grain was less than the opportunity cost of producing alternative crops
C.they were willing to sell excess grain when the demand for grain was greater than the opportunity cost of supplying more grain
D.they were willing to sell excess grain when the scarcity of grain decreased because they could sell it for a higher price