At a price of $8 per unit, the firm will sell 400 units, and we know that the marginal revenue of producing the 400th unit is $0 because for any straight-line demand curve, the associated marginal revenue curve has a horizontal intercept that is half the horizontal intercept of the demand curve. So if the firm maximizes its profit by charging $8 per unit, it must be the case that the marginal cost of the 400th unit is zero.