In a perfectly competitive industry, the industry supply curve is best conceptualized as Select one: O A. the vertical summation of the firms' ascending constant marginal cost curves O B. the vertical summation of the firms' identical marginal cost curves O C. the horizontal summation of the firms' identical marginal cost curves O D. as the vertical summation of buyers' willingness to pay O E. the horizontal summation of the firms' monopoly profit maximizing supply curve