To determine the duration it takes to empty the water tank based on the graph, we would typically look for the point where the water level reaches zero gallons. Since I don't have access to the image or graph you're referring to, I will suggest that you identify the x-axis value (representing time in hours) at which the water level (y-axis for gallons) reaches zero.
Once you identify that point on the graph, you can select the corresponding option. If you have any specific details from the graph, such as the coordinates of points or the slope of the line, feel free to share, and I can help further analyze it!