An oligopoist is correctly charging a price of $100 and is selling 400 units of output per day. If the firm increases price above $100, the demand curve is P=110 - 0.025Q. If the firm reduces price below $100, the demand curve becomes P=140 - 0.1Q. If the firm's marginal cost curve is horizontal, within what range could marginal cost vary without giving the firm an incentive to change the current price or quantity?
a. (110,140)
b. (55,70)
c. (60,90)
d. (80,120)