When approaching a railroad crossing, you should:

Answer: D — All of the above

Approaching a railroad crossing safely is important to avoid any accidents. Shifting into the lowest gear and stopping at least 15 feet away from the crossing are both important steps in this process. Shifting into the lowest gear allows for better control and slower speed while crossing the tracks. Stopping at least 15 feet away allows for a safe distance to assess the situation. Stopping and listening at the crossing can also be helpful, but it should not be the only action taken. Therefore, the most appropriate and complete course of action would be to do all of the above, as mentioned in option D.