A production possibility frontier that is a straight line sloping down from left to right suggests that:

A) more of both goods could be produced moving along the frontier.
B) the two products must have the same price.
C) the opportunity costs of the products are constant.
D) there are no opportunity costs.

A straight line PPF indicates that the opportunity cost of producing one good in terms of the other remains constant. For example, if a country could produce either guns or butter and the PPF is a straight line, producing one more unit of guns would always require the same amount of sacrifice in terms of how much butter must be given up. Therefore, the opportunity costs of the products are constant.