PPF is a straight line when

Select one:

a. the opportunity cost of one good in terms of another is constant and a mix of resources is used.

b. the opportunity cost of one good in terms of another is constant and only one resource is used with identical characteristics.

c. the opportunity cost of one good in terms of another is changing and a mix of resources is used.

d. the opportunity cost of one good in terms of another is changing and only one resource is used with identical characteristics.