2-6. You have been invited by friends to fly to Germany
for Octoberfest next year. For international travel,
you apply for a passport that costs $97 and is valid
for 10 years. After you receive your passport, your
travel companions decide to cancel the trip because
of “insufficient funds.” You decide to also cancel your
travel plans because traveling alone is no fun. Is your
passport expense a sunk cost or an opportunity cost?
Explain your answer. (2.1)