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