Paragraph: Two trains A and B are moving with speeds 20 m / s and...

Paragraph: Two trains $A$ and $B$ are moving with speeds $20 \mathrm{ m} / \mathrm{s}$ and $30 \mathrm{ m} / \mathrm{s}$, respectively in the same direction on the same straight track, with $B$ ahead of $A$. The engines are at the front ends. The engine of train $A$ blows a long whistle. Assume that the sound of the whistle is composed of components varying in frequency from $f_1=800 \mathrm{ Hz}$ to $f_2=1120 \mathrm{ Hz}$, as shown in the figure. The spread in the frequency (highest frequency - lowest frequency) is thus $320 \mathrm{ Hz}$. The speed of sound in still air is $340 \mathrm{ m} / \mathrm{s}$.Question: The distribution of the sound intensity of the whistle as observed by the passengers in train $A$ is best represented by