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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$.$ $$100$

b$.$ The value to Elena of the option she would have chosen had Option B not been available

c$.$ The average of the values to Elena of Options A$,$ C$,$ and D

d$.$ The value to Elena of Options A$,$ C$,$ and D combined