The amount of work required by the blower depends on what factors?

The amount of work required by the blower is significantly influenced by several factors, which makes option C the correct answer. This includes the capacity of the system, which pertains to how much air is being moved to achieve the desired heating or cooling. Additionally, the design and efficiency of the supply and return ducts play a critical role, as these components can create resistance to airflow. If the ducts are too small, poorly insulated, or not properly sealed, they can increase the workload on the blower as it has to work harder to overcome these obstacles.

The types of air filters used also contribute to the blower's work requirement. Filters that are too dense can restrict airflow, leading to increased strain on the blower as it tries to maintain the desired air circulation.

In contrast, the other options focus too narrowly on single aspects of the blower's operation. Blower size alone does not account for the various system dynamics at play, thermal load is only one of many considerations, and simply measuring the temperature difference overlooks crucial variables like duct design and filter impact. Thus, a holistic understanding of airflow, ductwork, and filtration is essential for determining the actual work required from the blower.