In which situation can the total population remain stable while its age structure changes?

Correct answer: Births, deaths and migration balance overall, but the shares of age groups change

The direct answer is A. Total population size and population structure are different ideas. Total size can remain unchanged when additions and losses balance each other, but the proportions of children, working-age people and older people may still change. For example, a fall in births can reduce the share of children, while longer survival can increase the share of older people. Migration can also bring mainly young adults or families and alter the age pattern without necessarily changing the final total. Option A is correct because it recognizes both balance in total numbers and change in composition. B is wrong because if nothing changes, the age structure cannot change. C is wrong because a change in area does not by itself alter the population’s age composition. D is unrelated to demographic structure. Memory cue: size means “how many”; structure means “who they are.”