Find the intervals on which f is increasing and decreasing. Superimpose the graphs of f and f' to verify your work. f(x)=x+4x Select the correct choice below and, if necessary, fill in the answer box(es) within your choice. OA. The function is increasing on and decreasing on (Simplify your answers. Type your answers in interval notation. Use a comma to separate answers as needed.) OB. The function is increasing on The function is never decreasing. (Simplify your answer. Type your answer in interval notation. Use a comma to separate answers as needed.) C. The function is decreasing on (-co,00). The function is never increasing. (Simplify your answer. Type your answer in interval notation. Use a comma to separate answers as needed.) OD. The function is never increasing nor decreasing.