Let the function M(t) denote the mass of bacteria culture (in mg) at time t days. The average rate of change of M(t) is 1.4 mg/day over the time interval [2, 7] and is 0.6 mg/day over the time interval [4, 7]. The average rate of change of M(t) over the time interval [2, 4] is 32 mg / day 57 mg / day O 6.6 2 mg / day 4.7 mg / day 5. 1 mg / day ONone of the above