A coal-burning electrical generating plant emits sulfur dioxide into the surrounding air. The concentration $C(x)$, in parts per million, is approximately given by the function $$C(x) = \frac{0.4}{x^2},$$ where $x$ is the distance away from the plant in miles.
The instantaneous rate of change of the sulfur dioxide concentration:
(A) 3 miles from the plant =
(B) 8 miles from the plant =