It's so noisy! "how can you stand it? " remarks caitlyn as highway traffic screams past her friend dave's ground-floor apartment. "i don't even notice it anymore," dave replies. this exchange best exemplifies the psychophysical concept of:

The correct answer is adaptation.

In terms of psycho-physiology, adaptation refers to the phenomenon wherein our sensory system shows decreased responsiveness to sensations or sensory stimuli after prolonged exposure to them. In this instance, when Dave says that he doesn't notice the sound of traffic outside his apartment anymore, it is because after prolonged exposure to the traffic noise, his auditory system (hearing) has adapted to it. 

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