Final answer:
Heavy sales of umbrellas during a rainstorm is an example of a causal relationship because the rain causes the demand for umbrellas to increase. The concept of a variable, which can change in value across different measurements, is key in understanding supply and demand.
Step-by-step explanation:
Heavy sales of umbrellas during a rainstorm are an example of a causal relationship. This is because the occurrence of the rainstorm causes the demand for umbrellas to increase - people need umbrellas to stay dry when it is raining. This relationship is one where a change in one variable directly causes a change in another variable, which makes it causal rather than just a correlation or coincidence. It is not a trend, because it is a response to a specific event rather than a long-term direction of change. It is also not a fad since that implies a short-term surge in popularity driven by fashion or novelty, rather than practical necessity.
The subject of the question that describes a variable is something whose value can change over multiple measurements, which is important when examining changes in demand caused by various factors such as weather conditions. A variable is a fundamental concept in both business and mathematics, having a direct impact on supply and demand dynamics in economic events.