A submarine is sitting still along the surface of the ocean. As a part of a drill, it dives at a constant speed to a certain number of feet below the surface of the ocean, and then travels for a time at that depth. When the drill is complete, it ascends at a constant speed to just below the surface and continues travelling at that depth. Which graph matches the description that compares time to the submarine's depth, in feet below the surface of the ocean, after it started its dive?