A firm's marginal product of labor is 4 and its marginal product of capital is 5. If the firm adds one unit of labor, but does not want its output quantity to change, the firm should:
选项：

A:use five fewer units of capital.
B:use 0.8 fewer units of capital.
C:use 1.25 fewer units of capital.
D:add 1.25 units of capital.