The population of a colony of bacteria is modeled by the function P(x) = 50(e^-x -e^-x^2)+10, for x is greater than or equal to 0, where population P is in thousands, x is in hours, and x=0 corresponds to the moment of introduction of a certain chemical into the colony's environment. At which time(s) is the bacteria population growing at an instantaneous rate of 0, going from positive growth just prior, to negative growth right after?
a) x=2.23498
b) x=.58746
c) x=.39382
d) x=.39382 and x=2.23498
e) x=1.71842