You and your friends meet at the mall for lunch. You've narrowed your choices down to a chicken sandwich or...

You and your friends meet at the mall for lunch. You've narrowed your choices down to a chicken sandwich or beef tacos. Since you only have enough money for one meal, you choose tacos.
Which concept is BEST illustrated in this scenario?
A. command economy
B. public good
C. opportunity cost
D. supply and demand