Concept:Caudal vertebrae are the vertebrae in the tail region, so their number indicates how developed the tail is.Explanation:Humans have a vestigial tail made of about 4 fused caudal vertebrae, known as the coccyx.Rats have a long tail that contains around 27 caudal vertebrae.Therefore, animal X with 4 caudal vertebrae is likely a human, and animal Y with 27 caudal vertebrae is likely a rat.Answer:D. humans and rat