Assuming that a period of stable population size is followed by a period when natality increases by ten (1O) per cent and immigration increases by twenty (2...

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If the population size is stable, it means that the birth rate (natality) and immigration rate are equal to the death rate (mortality) and emigration rate respectively. However, if there is a ten percent increase in natality and a twenty percent increase in immigration, the population size will begin to increase. This is because the number of individuals added to the population through births and immigration will exceed the number lost through deaths and emigration, resulting in a net increase in population size. Therefore, the correct answer is "begin to increase."