The “Doppler effect” refers to the perceived change in pitch that occurs when the source of a sound is in motion relative to the observer. For example, the siren on a passing police car will sound higher than its true pitch as the car approaches, sound the same as its true pitch as the car passes, and sound lower than its true pitch as the car travels away from the observer.

If two trains pass each other going opposite directions on parallel east-west tracks, which of the following observations provides another illustration of the effect described above?
If the eastbound train blows its horn as they pass, passengers on the westbound train will hear a sound that decreases in pitch.
If the eastbound train blows its horn as they pass, passengers on the westbound train will hear a sound that increases in pitch.
If the eastbound train blows its horn as they pass, passengers on the eastbound train will hear a sound that decreases in pitch.
If the eastbound train blows its horn as they pass, passengers on the eastbound train will hear a sound that increases in pitch.
If the eastbound train blows its horn as they pass, passengers on the eastbound train will hear a sound that is steady in pitch.