The supply curve found by taking the horizontal summation of the short-run supply curves of all of the firms in a perfectly competitive industry is called the _____ curve.

A) marginal cost
B) short-run market supply
C) interim market supply
D) competitive

This is because the short-run market supply curve represents the quantity of output that all firms in the industry are willing and able to produce at each price level in the short run, and taking their horizontal summation gives the market supply curve. Choices A, C, and D are incorrect because they do not accurately describe the supply curve that is found in a perfectly competitive industry.