For the function below, find (a) the critical numbers; (b) the open intervals where the function is increasing; and (c) the open intervals where it is decreasing.\\
$f(x) = \frac{2}{3}x^3 - 4x^2 - 24x - 12$\
(a) Determine the critical numbers. Select the correct choice below and, if necessary, fill in the answer box to complete your choice.\
A. The critical number(s) is/are\
(Type an integer or a simplified fraction. Use a comma to separate answers as needed.)\
B. There are no critical numbers.