The Least-Cost Rule Is That the Ratio of the Marginal

The least-cost rule is that the ratio of the marginal product of a factor to the price of that factor should be:

A) equal to the amount of the factor.
B) proportional to the amount spent on the factor.
C) equal for all factors if the firm is a perfect competitor.
D) equal to the price of the product.
E) equal for all factors, no matter whether the firm sells in a perfectly or an imperfect competitive market.