Which of the following best describes the short-run supply curve for an individual perfectly competitive firm? Select one: O a. It is the vertical axis at prices less than minimum average variable cost and is the firm's marginal cost curve at prices above minimum average variable cost. O b. It is the upward-sloping part of the firm's marginal cost curve. O c. It is the firm's marginal cost curve. O d. It is the vertical axis at prices less than minimum average total cost and is the firm's marginal cost curve at prices above minimum average total cost