When can local and national effects differ in population change?

Correct answer: When internal migration changes local population but not national total

The direct answer is A. Local and national population effects can differ because internal migration redistributes people within one country. Step 1: a person or family leaves District X, so District X loses population. Step 2: the same person or family enters District Y, so District Y gains population. Step 3: the country has not gained or lost anyone merely through this internal movement, so the national total remains unchanged, assuming births, deaths and international migration are not involved. Option A correctly describes this difference in scale. B does not explain a local increase or decrease and is not required for the distinction. C removes the process that creates the difference. D is an extreme condition and does not explain the demographic relationship. Internal migration changes distribution, while births, deaths and international migration can change the national total. Memory cue: movement inside a country changes “where people live,” not necessarily “how many people the country has.”