The Stone Company produces three sizes of window fans: small, medium, and large. The manager has formulated an LP model for fan production: maximize 6 x1 + 8 x2 + 5 x3 (profit) subject to Labor 3 x1 + 4 x2 + 5 x3 ? 160 hours Metal 1 x1 + 2 x2 + 3 x3 ? 100 pounds Plastic 2 x1 + 2 x2 + 2 x3 ? 110 pounds Large fan x3 3 ? 18 x1, x2, x3 ? 0 Briefly explain or define each of these parts of the model: d. The terms labor, metal, and plastic