The total number of bacteria (in millions) present in a culture is given by N(t) = 2t(5t + 9)^1/2 + 12, where t represents time (in hours) after the beginning of an experiment. Find and interpret the rate of change of the population of bacteria with respect to time for the following numbers of hours.
(a) 0 

(b) 7/5 

(c) 8