A loudspeaker sends a sound wave into a tube that is at first completely filled with water. A tap at the base of the tube is then opened, allowing the water to drain away slowly. At certain air-column lengths in the tube, the sound that is heard becomes much louder. The first loud sound occurs when the air column in the tube is of length $x$. The next loud sound occurs when the air column in the tube is of length $y$. What is the wavelength of the sound wave from the loudspeaker?
- A$2x$
- B$4y$
- C$2(y - x)$
- D$4(y - x)$