The profit function represents a
(a) straight line
(b) a parabola with vertex at $(20,700 / 3)$
(c) a parabola with vertex at $\left(\frac{50}{3}, \frac{700}{3}\right)$ and focus $\left(\frac{50}{3}, \frac{2199}{3}\right)$
(d) a parabola with vertex at $\left(\frac{50}{3}, \frac{700}{3}\right)$ and focus $\left(\frac{50}{3}, \frac{2099}{3}\right)$