Answer:
a curved line; diminishing marginal returns
Step-by-step explanation:
The specific factor model is one that assumes that a country produces two goods using two factors of production in a perfectly competitive market. That is labour and and capital.
The production possibility frontier is defined as the maximum combination of two products that can be produced by a country. The PPF tends to be curved because of the law of diminishing returns. As more of one factor of production is added it will result in reduced output of the product over time.