A bacterial culture starts with 200 cells. For the first 2 hours the population grows exponentially with constant rate k1=0.4 (per hour). Immediately afterward, a nutrient change reduces the growth rate to a new constant k2=0.1 (per hour). What is the closest integer value of the population after a total of 6 hours?
a) 574 cells
b) 664 cells
c) 811 cells
d) 445 cells