What should be seen together while studying population change?

Correct answer: Birth, death and migration

The direct answer is A: birth, death, and migration should be studied together. Population change is not explained by only one event. Births add people to a population, deaths remove people, and migration transfers people into or out of a place. In simple terms, total change is influenced by births plus in-migration, minus deaths and out-migration. A is correct because it includes all three main demographic processes. B, rainfall, may influence settlement or farming in some situations but is not itself the complete set of population-change processes. C, mountains, are physical features and cannot by themselves explain population change. D, rivers, may influence where people live but are not the full demographic explanation. A useful memory cue is “add births, subtract deaths, track movement.” Studying all three gives a more complete understanding of why population size changes.