When Economists Talk About a Barrier to Entry, They Are

When economists talk about a barrier to entry, they are referring to

A) a factor that makes it difficult for potential competitors to enter a market.
B) the opportunity cost of equity capital that is incurred by a firm producing at minimum total cost.
C) the downward-sloping portion of the long-run average total cost curve.
D) the declining output experienced as additional units of a variable input are used with a given amount of a fixed input.