If a certain combination of goods or services lies outside the production possibilities curve of an economy, which of the following is true?
a. Effective trade barriers have reduced foreign imports into the economy.
b. New technology is being used in production.
c. Resources are not available to achieve that combination of goods or services.
d. Resources are not being used efficiently to achieve that combination of goods or services.
e. Resources are being used at a more rapid rate than they were in the past