Using to the fit formula, how much vigorous activity should you be doing a week?

The FITT acronyms stands for Frequency, Intensity, Time and Type of exercise. 
FITT formular is normally used in order to encourage individuals, especially children and teenagers to stay physically fit. Using the FITT formula, one will have to participate in vigorous activities about three to five days in a week. To be effective, the intensity of the exercise chosen must not be too easy or too hard.

Answer(d) they all send signals from the brain to the body through long axons in the vertebral column;