Contrast. there's a contrast between joyful and despondent.
hope it helps

The harmonic frequency refers to the resonant frequency that is an integral multiple of the fundamental frequency, harmonics are usually simple sinusoidal oscillation at a single frequency. where as Resonance frequency are frequencies that are not necessarily sinosudal or integral multiple of fundamental frequency and it may have complex oscillations made up of multiple harmonics ~
It's like if you play a piano with a single key, it will show harmonic frequency, but if you use multiple keys at a time it may show Resonance.
Answer: It reveals Nancy Lee’s nervousness to interact with her principal
Explanation: Nancy Lee was worried that she was failing chemistry. Therefore, she was afraid she wasn’t doing adequately in school, which could devastate her role as an honors student and possibly even the Artist Prize for the local art school she wanted to go to.
It is the old man is attempting to catch the fish.