Why rising portion of MC curve is supply curve of firm??

 A perfectly competitive firm's supply curve is that portion of its' marginal cost curve that lies above the minimum of the average variable cost curve. A perfectly competitive firm maximizes profit by producing the quantity of output that equates price and marginal cost. As such, the firm moves along it's marginal cost curve in response to alternative prices. Because the marginal cost curve is positively sloped due to the law of diminishing marginal returns, the firm's supply curve is also positively sloped.