A small source produces spherical waves. The wave intensity $I$ at a point $P$, which is a distance $r$ from the source, varies inversely as $r^2$. What relationship exists between the wave amplitude $a$ and the distance $r$?
- A$a^2 \propto \frac{1}{r}$
- B$a \propto \frac{1}{r}$
- C$a \propto \frac{1}{r^2}$
- D$a \propto \frac{1}{r^4}$