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