Ben quit his job as an economics professor to become a golf professional. He gave up his $30,000 salary and invested his retirement fund of $50,000 (which was earning 10 percent interest) in this venture. After all expenses, his net winnings were $35,000. Ben's economic profits were:________.
a. $35,000.
b. $5,000.
c. $2,000.
d. zero.