A boat has a ladder that has six rungs; each rung is one foot apart. The bottom rung is one foot from the water. The tide rises at 12 inches every 15 minutes. High tide peaks in one hour. When the tide is at its highest, how many rungs are under water?

This is a physics riddle about relative motion. As the tide rises, the boat floats upward with the water. Since the boat is floating, it rises at the same rate as the tide. Therefore, the ladder rungs maintain the same position relative to the water level, and no additional rungs go underwater. The answer is None (option D).