Give scientific reasons: Indole-$3$-acetic acid $(IAA)$ is not the only naturally occurring auxin.

(N/A) Auxins are a class of plant hormones that promote growth. While Indole-$3$-acetic acid $(IAA)$ is the most common and primary natural auxin,it is not the only one. Other chemical substances,such as $4$-chloro $IAA$ and phenylacetic acid $(PAA)$,have been isolated from various plant species. These compounds exhibit auxin-like physiological effects,such as cell elongation and root initiation,and are therefore classified as naturally occurring auxins.