The $($absolute$)$ value of the slope of a production isoquant $($equal$-$product curve$)$ is equal to the

a$.$

amount of one input used divided by the amount of the other input.

b$.$

ratio of the marginal utilities of the two inputs.

c$.$

ratio of the marginal products of the two inputs.

d$.$

ratio of the marginal costs of the two inputs.