When Isabel visited the children's zoo, she heard squeals coming from the baby pigs and loud grunts from the adult pigs. Her perception of the high-pitched squeals can be explained by the theory of hearing, and her perception of the low-pitched grunts can be explained by the theory of hearing.

Place - Frequency.

Explanation:

As in the example given, according to the Place theory of hearing explains the high pitched squeals given that, when a sound exceeds the frequency of 5,000 hertz, it causes a vibration in our hearing system that allows us to listen to it. On the other hand, the frequency theory of hearing explains how she hears the low pitched grunts given that, as it is of a lower tone, the vibrations in our hearing system are slower (around 700 hertz or lower) and this allows us to detect the specific sound.