The graph below shows the results of a laboratory investigation which measured the body temperatures of a lizard and a bird under changing artificial conditions. Use it to answer the question.

Which of the statements below is valid?

• A The bird's blood was always warmer than that of the lizard
• B The body temperature of the bird varied less than that of the lizard during changes in environmental temperature
• C The body temperature of the bird remained constant despite changes in environmental temperature
• D The body temperature of the lizard was always close to that of the environmental temperature