To Derive the Equation for an Isocost Line When the Level

To derive the equation for an isocost line when the level of capital is plotted on the vertical axis and the level of labor is plotted on the horizontal axis, you should:

A) solve the production function for $K$ .
B) solve the production function for $L$ .
C) solve the total cost equation for $K$ .
D) solve the total cost equation for $L$ .