<P> An outward shift of the PPF results from growth of the availability of inputs, such as physical capital or labour, or from technological progress in knowledge of how to transform inputs into outputs . Such a shift reflects, for instance, economic growth of an economy already operating at its full productivity (on the PPF), which means that more of both outputs can now be produced during the specified period of time without sacrificing the output of either good . Conversely, the PPF will shift inward if the labor force shrinks, the supply of raw materials is depleted, or a natural disaster decreases the stock of physical capital . </P> <P> However, most economic contractions reflect not that less can be produced but that the economy has started operating below the frontier, as typically, both labor and physical capital are underemployed, remaining therefore idle . </P> <P> A PPF (production possibility frontier) typically takes the form of the curve illustrated above . An economy that is operating on the PPF is said to be efficient, meaning that it would be impossible to produce more of one good without decreasing production of the other good . In contrast, if the economy is operating below the curve, it is said to be operating inefficiently because it could reallocate resources in order to produce more of both goods or some resources such as labor or capital are sitting idle and could be fully employed to produce more of both goods . </P> <P> For example, if one assumes that the economy's available quantities of factors of production do not change over time and that technological progress does not occur, if the economy is operating on the PPF, production of guns would need to be sacrificed to produce more butter . If production is efficient, the economy can choose between combinations (points) on the PPF: B if guns are of interest, C if more butter is needed, D if an equal mix of butter and guns is required . </P>

Which of the following is the most likely to be a variable factor of production at a university