Which statement is true? shift I. A negative productivity shock to all goods and services can be represented as a inwards of the Production Possibilities Frontier. II. A positive productivity shock to all goods and services can be represented as a shift inwards of the Production Possibilities Frontier. III. The slope of the production possibilities frontier is the opportunity cost of producing an additional unit of a good in terms of the other good. a. only I and III are true b. only I is true c. only II and III are true only III is true e. only II is true d.