Final answer:
A line graph is the best choice for showing how the relationship between two variables, such as time and the unemployment rate, changes over time. It uses a horizontal axis for the independent variable and a vertical axis for the dependent variable.
Step-by-step explanation:
The graph that effectively illustrates how a relationship between two variables changes over time is a line graph. This type of graph displays data points connected by straight lines, which makes it easier to observe trends and changes. In a line graph, one axis usually represents the independent variable, often time, while the other axis represents the dependent variable, showing how its value fluctuates in relation to the independent variable. An example of using a line graph is to show the unemployment rate over a period of time. Here, the time is plotted on the horizontal axis (x-axis), while the unemployment rate is plotted on the vertical axis (y-axis). As time progresses, we can see the changes in the rate clearly, allowing us to discern patterns or trends from the data presented.