Figure $13-18$

Refer to Figure $13-18.$ Which of the following statements is true?

$D_a$ represents the long$-$run demand curve facing a monopolistic competitor in a constant$-$cost industry while $D_b$ depicts the demand curve in the short run.

$D_2$ represents the long$-$run supply curve in a perfectly competitive, constant$-$cost industry while $D_b$ depicts the long$-$run demand curve facing a monopolistic competitor in a decreasing$-$cost industry.

$D_2$ represents the long$-$run demand curve facing a monopolistic competitor in a constant$-$cost industry while $D_b$ depicts the long$-$run demand curve in an increasing$-$cost industry.

$D_a$ represents the long$-$run demand curve facing a perfect competitor while $D_b$ depicts the long$-$run demand curve facing a monopolistic competitor.