Snow began falling at noon (t = 0) on Sunday (no snow on the ground). The height (measured in inches) of snow on the ground at a certain location can be modeled by a polynomial function P(t), with independent variable t time (measured in days). On the following Sunday at noon (t=7) the snow had completely melted away. Explain why there has to be a time between the two Sundays such that P'(t) = 0