When determining the production possibilities curve

a. the trade-off between the goods in the economy remains constant.

b. the prices of the goods are used.

c. the amount of productive resources remains constant.

d. the prices of resources are used?

So when we are determining the production possibilities curve, the amount of productive resource remain constant or at least an assumption is made that the amount of resources is fixed while deriving the curve. This is done that way because to avoid fluctuations in the curve while analyzing the curve.