Final answer:
In Figure A8 (c), the bars on the graphs illustrate the percentage of males and females in each age group, using a vertical axis that measures percentages, allowing the population proportions across different age brackets to be compared over the years. The correct answer is option 4.
Step-by-step explanation:
Bar graphs are versatile tools for representing data in a visually accessible way. When analyzing Figure A8 from the provided information about U.S. population data, it becomes clear that bar graphs can convey different types of data depending on their format. In the case of Figure A8 (c), we see the vertical axis set as a measure of percentages rather than the actual number of persons. This is a strategic move to allow comparison of the population proportions between different age groups across the years, represented as 1970, 2000, and 2030.
Looking at the specifics, Figure A8 (a) displays three bars for each year, showing the total number of persons within each age bracket. Figure A8 (b) simplifies this by using a single bar for each year but visually separates the age brackets through shading within the bar. The key distinction comes in Figure A8 (c), where every bar represents the entire population as 100%, divided into segments that correspond to the percentage of population within each age bracket.
In summary, the bars on the graphs in Figure A8 (c) represent the percentage of males and females in each age group, providing an illustrative way to observe how population distribution shifts over time. Hence, the correct option that the bars on the graphs represent is option 3, which is the percentage of males and females in each age group.