Which statement best describes what the bars illustrate?

• A. The numbers of births per year
• B. The distribution by age and sex across the population
• C. The total population size at each time point
• D. The numbers of people added to the population annually in each decade

Answer: (D)

Bars on a population-time graph are usually showing how much the population changes from one point to the next—the additions during that interval. In other words, the height of each bar represents the number of people added to the population in that period (births plus arrivals minus deaths and departures). This is why the statement describing the bars as numbers added to the population during each decade fits best. If the chart were showing total population size at each time point, the bars would reflect absolute totals rather than changes between points, and a population pyramid would illustrate distribution by age and sex rather than time-based additions. Similarly, a chart of births per year would focus on births alone rather than net change over each interval.