Assume a firm produces 500 units of a good by using two inputs, capital and labor, whose per unit prices are $10 and $4. Assume also that the marginal physical product of the last unit of capital is 30 and the marginal physical product of the last unit of labor is 10. To minimize costs this firm should employ which of the following?

A. the existing combination of resources

B. more labor and less capital

C. both more labor and more capital

D. more capital and less labor