You Can Now Sell 120 Cups of Lemonade at 40

Multiple Choice

You can now sell 120 cups of lemonade at 40 cents per cup, but demand is dropping at a rate of 6 cups per week each week. Assuming that raising the price doesn't affect the demand, how fast do you have to raise your price if you want to keep the revenue constant ?

A) 6 cents per week
B) 2 cents per week
C) 1 cent per week
D) 4 cents per week
E) 5 cents per week