Read this passage from The Phantom Tollbooth. The sky became quite gray and, along with it, the whole countryside seemed to lose its color and assume the same monotonous tone. Everything was quiet, and even the air hung heavily. The birds sang only gray songs and the road wound back and forth in an endless series of climbing curves.

What literary device does the author use to make the setting drearier?

a homophone
an idiom
onomatopoeia
a pun