Let a Firm Use Labor $( L )$ And Capital $( K )$

. Let a firm use labor $( L )$ and capital $( K )$ as its only inputs to produce an output, $Q$ . The cost of labor is $w = \$ 5$ per labor hour and the cost of capital is $r = \$ 15$ per machine hour. When capital is measured on the vertical axis and labor on the horizontal axis, what is the slope of an isocost line for this firm?

A) $- 3$ .
B) $- 5$ .
C) $- 15$ .
D) $- 1 / 3$ .