Dr. difonzo notices several students nodding in agreement as he lectures. subsequently, his rhetoric becomes more confident and more passionate. the students have provided reinforcement.

I think the students have provided : Positive reinforcement

By nodding during his lectures, Dr. Difonso will get the perception that students pay attention to what he's talking about.
By getting this, Dr. Difonso will fell more valued and validated by his students, and this will lead to more passion for his works.

Complementary goods. they go together, thats why one price increase and the others demand increases