At the beginning of the year, Hank puts $100,000 into an investment.  At the end of the year, the investment is completed and he is given a check for $110,000. Meanwhile, Susan puts $100,000 in another investment, and receives $140,000.  In retrospect, what was Hank's opportunity cost?

a. $30,000

b. $40,000

c. $100,000

d. $110,000