Read this passage from the phantom tollbooth.

"r-r-r-g-h-r-o-r-r-h-f-f," exclaimed the watchdog as he dashed up to the car, loudly puffing and panting.

milo's eyes opened wide, for there in front of him was a large dog with a perfectly normal head, four feet, and a tail--and the body of a loudly ticking alarm clock.

what type of wordplay does the author use in the passage?

homophone
idiom
onomatopoeia
pun