Asound recording is made of a train as it blows its whistle for 10 seconds. the pitch of the sound appears to be decreasing during the 10 seconds. what conclusion, if any, could be made about the motion of the train and how do you justify this conclusion?

a) applying the doppler effect. the train is moving away from the recording device because the frequency is decreasing.
b) applying the doppler effect. the train is slowing and moving toward the recording device because the frequency is decreasing.
c) applying the doppler effect. the train is slowing but the direction cannot be determined because the frequency is decreasing.
d) applying the doppler effect. the train is moving at constant speed toward the recording device because the frequency is decreasing.