A company manufactures x smartphones which sell for $r and y portable speakers which sell for $s. The weekly demand and cost equations are as follows: r=-5x+3y+270 s 2x-y+339 C(x, y) = 120 x + 290 y + 360 At a production level of 79 smartphones and 90 portable speakers, what is the marginal profit associated with selling portable speakers? a) -$190 per portable speaker b) $264 per portable speaker c) $341 per portable speaker d) -$355 per portable speaker