Spending on fiber-optic links us telephone company spending

Question

Question: Spending on fiber-optic links u.s. Telephone company spending on fiber-optic links to homes and businesses from 2001 to 2006 is projected to be

S(t) = -2.315t³ + 34.325t² + 1.32t + 23           (0 ≤ t ≤ 5)

Billion dollars in year t, where t is measured in years with t = 0 corresponding to 2001. Show that S'(t) > 0 for all t in the interval [0, 5]. What conclusion can you draw from this result?