Please help with this question

12. A PPC is a straight line as a result of

a. Choice.

b. Increasing opportunity costs.

c. Decreasing opportunity costs.

d. Scarcity.

e. Constant opportunity costs.

The existence of an externality is proof that there is a(n):

a. Undervaluation of a good.

b. Undervaluation of a cost.

c. Property dispute.

d. market failure