If an Economy Is Operating on Its Production Possibilities Frontier

Multiple Choice

If an economy is operating on its production possibilities frontier (PPF) , are there any unemployed resources in the economy?

A) Yes, because if there weren't any unemployed resources the economy would be producing beyond its PPF.
B) No, because if there were any unemployed resources the economy would be producing below its PPF.
C) It depends on whether the economy's PPF is a concave (downward-sloping) curve or a straight line.
D) Yes, because there are always some natural resources that are unemployed.