Fred's demand schedule for movie DVDs is as follows: At $60, he would buy 1; at $50, he would buy two; at $30, he would buy 3; and at $20, he would buy 4. If the price of movie DVDs equals $40, the consumer surplus Fred receives from purchasing movie DVDs would be:
a. $40.
b. $180.
c. $20.
d. $30.