A Production Possibility Curve (PPC) is a graphical representation of the different combinations of two goods that can be produced using the available resources efficiently. The shape of the PPC depends on the opportunity cost of producing one good in terms of the other good.

If the opportunity cost of producing a good remains constant, the PPC will be a straight line. This is because the constant opportunity cost implies that the resources used to produce the goods are perfectly substitutable. In other words, resources can be easily shifted from one good to another without any loss in productivity.

The Straight Line PPC:
- Represents constant opportunity cost: The slope of the PPC is equal to the opportunity cost of producing one good in terms of the other. A straight line indicates that the slope remains constant, which means that the opportunity cost is constant as well.
- Perfect substitutability of resources: In this scenario, resources can be easily shifted from one good to another without any loss in productivity. This is the reason why the opportunity cost remains constant throughout the PPC.
- Equal trade-offs: A straight line PPC also signifies that there are equal trade-offs between the two goods. This means that when an economy decides to produce more of one good, it needs to give up the same quantity of the other good. This is in contrast to a convex or concave PPC, where the trade-offs between goods are not equal.

In conclusion, if the opportunity cost is constant, the PPC would be a straight line. This is because the constant opportunity cost represents perfect substitutability of resources, which allows for equal trade-offs between the two goods throughout the PPC.