In the models that describe population growth, r stands for
a) carrying capacity
b) population density
c) a time interval
d) per capita population growth rate
e) total number of individuals in the population

d) per capita population growth rate

Explanation:

Per capita growth rate of a population is symbolized as "r" in population growth models. Here, "r" is the rate of change of a population on a per capita basis. The change in the population includes both an increase and a decrease in its size.  The per capita growth rate is calculated by subtracting the birth rate of the population from its death rate. Therefore, r= b-d. Here, "b" is the birth rate of the population and "d" is the death rate.

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this age may represent the age of the earth's accretion, of core formation, or of the material from which the earth formed. this dating is based on evidence from radiometric age-dating of meteorite material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.