Answer:
the availability of scarce resources needed for production
Explanation:
The production possibilities frontier model - PPF shows how much an economy can produce of two different products typically showing production goods such as machinery and consumption goods as donuts, there is a tradeoff between the products along the curve and any point in which this trade off occur is efficient.
The only ways to shift the curve outward or inward is by a change on technology that affect both of the goods, by trade, or by the availability scarcity of the resources needed to produce. those goods