According to the Doppler effect, when a train comes to us, what will his flute sound like?

According to the Doppler effect, when the vibration frequency of an object is constant, the farther away from the sound source, the lower the tone of the sound, and the closer to the sound source, the higher the tone of the sound.

When someone stands by the train track and the oncoming high-speed train honks, he will hear the sound with higher and higher frequency.

So choose: a