Which of the Following Is a Plausible Example of a Doppler

Which of the following is a plausible example of a Doppler blueshift?

A) A star appears to have a much higher temperature when moving toward the Earth than when moving away.
B) An ambulance's siren changes to a higher pitch as it speeds toward you.
C) A star's colour becomes redder as it moves away from the Earth.
D) A moving train's whistle shifts to a frequency so high that humans can't hear it.