Which point in this graph shows the beginning of a period of exponential growth?
С
Population size-
B
Time
O A. Point C
ОО
B. Point D
C. Point B
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C. Point B

Explanation:  

The exposed curve represents a logistic growth model. Population growth depends on density. Natality and mortality depend on the population size, meaning that there is no independence between population growth and population density.

When a population grows in a limited space, density rises gradually and eventually affects the multiplication rate. The population per capita growth rate decreases as population size increases. The population reaches a maximum point delimited by available resources, such as food or space. This point is known as the carrying capacity, K.

K is a constant that equals population size at the equilibrium point, in which the natality and the mortality rate get qual to each other.

Referring to population size as N, when

N<K, the population can still grow. N approximates to K, the population´s growth speed decreases. N=K the population reaches equilibrium, N>K, the population must decrease in size because there are not enough resources to maintain that size.  

The sigmoid curve represents the logistic growth model.

Point A: During this period, the population size is relatively stable. Little growth is expressed. Point B: At the beginning of this period, population growth increases softly. The more individuals are in the population, the more that reaches sexual maturity and get to reproduce, leaving fertile offspring. This fertile offspring also get to reproduce and leave more fertile individuals. At the end of the period, the curve shows a sharp slope, reflecting the significant increase in the population reproductive rate.  Point C: At the beginning of this period, the population keeps on growing exponentially. But at the end of the period, the curve tends to stabilize, meaning that the population stops growing in size so fast. The population slow and gradually reaches the equilibrium point, K.Point D: The population got to stabilize.