Which of the following can cause a limit control to open?

A limit control is a safety device used in heating systems to monitor temperature and ensure that conditions do not exceed safe operational limits. When the temperature exceeds a predetermined value, the limit control will open to prevent overheating, which could lead to equipment damage or hazardous situations.

When considering the elements that can lead to a limit control opening, a dirty filter can restrict airflow, causing the system to overheat since it cannot effectively circulate air. Similarly, when there is too high of supply-air static pressure, it indicates an obstruction or imbalance in the airflow, which can also lead to the system running hotter than intended. Lastly, insufficient return air reduces the overall efficiency of the heating system, again contributing to overheating.

Each of these factors—whether it’s restricted airflow from a dirty filter, high static pressure, or inadequate return air—can compromise the heating system's ability to maintain safe operating temperatures. Therefore, all of these situations can indeed cause a limit control to open, making the choice that encompasses all these scenarios the correct response for this question.