A company's cost, in dollars, to produce x units is modeled by C(x) = 3x² - 60x + 800. How many units should be produced to minimize cost?

The cost function $C(x) = 3x^2 - 60x + 800$ is an upward-opening parabola, so its minimum is at the vertex: $x = -\frac{b}{2a} = -\frac{-60}{2(3)} = 10$ units, choice D. The minimum cost is $C(10) = 300 - 600 + 800 = 500$ dollars. Choice A (20) comes from dividing by $a$ instead of $2a$. Choice B (60) is the coefficient of $x$. Choice C (800) is the fixed cost, the value of $C(0)$. The question asks how many units to produce, which is the $x$-coordinate of the vertex.