If a Perfectly Competitive Firm Can Acquire the Services of as Much

If a perfectly competitive firm can acquire the services of as much capital as it wishes at a constant rental rate of r per year, it should employ capital up to the point at which the

A) Rental rate is equal to variable cost
B) Rental rate is equal to marginal cost
C) Marginal product of capital is equal to r
D) Value of the marginal product of capital is equal to r