A purely competitive firm's short-run supply curve is:

A) perfectly elastic at the minimum average total cost.
B) upsloping and equal to the portion of the marginal cost curve that lies above the average variable cost curve.
C) upsloping and equal to the portion of the marginal cost curve that lies above the average total cost curve.
D) upsloping only when the industry has constant costs.

In the short run, a purely competitive firm's supply curve is equal to the portion of the marginal cost curve above the minimum of the average variable cost curve. This is because in the short run, firms cannot adjust their fixed costs and must continue to produce as long as their price exceeds their variable costs. Therefore, any price above the minimum of the AVC will induce firms to produce, and their supply curve will be upsloping and equal to the portion of the MC curve above the AVC curve.