The first figure is correct, and the other two figures can't distinguish the definition of marginal benefit. Marginal benefit is unit benefit, that is, the benefit or income obtained by increasing the output or sales volume of a unit. The price P is fixed, and the sales volume Q is always changing, so the marginal benefit here is the marginal benefit, that is, the income obtained by the increase of sales volume, that is, R'(Q), not R'(P). If the sales q is