${l_1} = \frac{{{\lambda _1}}}{4} \Rightarrow {\lambda _1} = 4{l_1}$ and ${l_2} = \frac{{{\lambda _2}}}{2} \Rightarrow {\lambda _2} = 2{l_2}$
Given ${n_1} = {n_2}$ so $\frac{v}{{{\lambda _1}}} = \frac{v}{{{\lambda _2}}} $
$\Rightarrow \frac{v}{{4{l_1}}} = \frac{v}{{2{l_2}}} = \frac{{{l_1}}}{{{l_2}}} = \frac{1}{2}$
$(A)$ $y(t)=A \sin \frac{\pi x}{6} \cos \frac{50 \pi t}{3}$
$(B)$ $y(t)=A \sin \frac{\pi x}{3} \cos \frac{100 \pi t}{3}$
$(C)$ $y(t)=A \sin \frac{5 \pi x}{6} \cos \frac{250 \pi t}{3}$
$(D)$ $y(t)=A \sin \frac{5 \pi x}{2} \cos 250 \pi t$

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 $340 \mathrm{~m} / \mathrm{s}$.
$1.$ The speed of sound of the whistle is
$(A)$ $340 \mathrm{~m} / \mathrm{s}$ for passengers in $A$ and $310 \mathrm{~m} / \mathrm{s}$ for passengers in $B$
$(B)$ $360 \mathrm{~m} / \mathrm{s}$ for passengers in $A$ and $310 \mathrm{~m} / \mathrm{s}$ for passengers in $B$
$(C)$ $310 \mathrm{~m} / \mathrm{s}$ for passengers in $A$ and $360 \mathrm{~m} / \mathrm{s}$ for passengers in $B$
$(D)$ $340 \mathrm{~m} / \mathrm{s}$ for passengers in both the trains
$2.$ The distribution of the sound intensity of the whistle as observed by the passengers in train $\mathrm{A}$ is best represented by
$Image$
$3.$ The spread of frequency as observed by the passengers in train $B$ is
$(A)$ $310 \mathrm{~Hz}$ $(B)$ $330 \mathrm{~Hz}$ $(C)$ $350 \mathrm{~Hz}$ $(D)$ $290 \mathrm{~Hz}$
Give the answer question $1,2$ and $3.$