A sound source with constant power $P$ is placed in open space. The sound intensity $I$ at a distance $r$ from this source is given by $I = \dfrac{P}{4\pi r^2}$. In what way does the amplitude $a$ of the vibrating air molecules change as the distance $r$ from the source increases?
- A$a \propto \dfrac{1}{r}$
- B$a \propto \dfrac{1}{r^2}$
- C$a \propto r$
- D$a \propto r^2$