22. The production possibilities curve can be used to illustrate the economic concepts of _____________ (resource combinations outside the production possibilities curve are unattainable), _____________ (selecting among the alternative bundles available along the production possibilities curve), _____________ (how much of one good you give up to get another unit of the second good as you move along the production possibilities curve), _____________ (being on the production possibilities curve rather than inside it), and _____________ (shifting the production possibilities curve outward).