问一道GMAT逻辑题~

问一道GMAT逻辑题~
The “Doppler effect” refers to the universally perceived change in the apparent pitch of a sound when that sound is approaching or receding from the listener.Specifically,whenever the distance between the listener and the source of the sound is decreasing,the sound will be perceived by the listener as higher than its true pitch; whenever that distance is increasing,the sound will be perceived by the listener as lower than its true pitch.If the distance between the listener and the source is not changing,the listener will perceive the true pitch of the sound.
If the above principle holds,which of the following should be observed as an eastward-traveling train is approached by a westward-traveling train that is blowing its horn?
\x05\x05\x05\x05\x05\x05A.Passengers in the eastbound train should hear the true pitch of the horn; passengers in the westbound train will hear a sound lower than the true pitch.
\x05\x05\x05\x05\x05\x05B.Passengers in the westbound train should hear the true pitch of the horn; passengers in the eastbound train will hear a sound lower than the true pitch.
\x05\x05\x05\x05\x05\x05C.Passengers in the eastbound train should hear the true pitch of the horn; passengers in the westbound train will hear a sound higher than the true pitch.
\x05\x05\x05\x05\x05\x05D.Passengers in the westbound train should hear the true pitch of the horn; passengers in the eastbound train will hear a sound higher than the true pitch.
\x05\x05\x05\x05\x05\x05E.Passengers in both trains should hear a sound that is higher than the true pitch.
为什么选D?解释一下题意和选项的意思吧