10. Assume that at the current market price of $4 per unit of a good, you are willing and able to buy 20 units. Last year at a price of $4 per unit, you would have purchased 30 units. What has most likely happened over the last year?
[A] Supply has decreased.
[B] Quantity supplied has decreased.
[C] Demand has decreased.
[D] Supply has increased.
[E] Demand has increased.