Elena's aunt gave her $100 for her birthday with the condition that Elena buys herself something. In deciding how to spend the money, Elena narrows her options down to four choices:

Option A, Option B, Option C, and Option D. Each option costs $100. Finally, she decides on Option B. The opportunity cost of this decision is:________

a. the value to Barb of Options A, C and D combined.
b. the value to Barb of the option she would have chosen had Option B not been available.
c. the average of the values to Barb of Options A, C, and D.
d. $100.