because an age-sex pyramid with a very wide base indicates a high birth rate and a rapidly growing, youthful population. When a country successfully implements measures to reduce population growth, the birth rate begins to fall. This reduction directly shrinks the relative size of the youngest age group (the base of the pyramid) over time. Therefore, pyramid A (which has a narrower, slightly trimmed base compared to B) represents the population 20 years later (more recent), reflecting a slight reduction in the growth rate due to the implemented control measures.