$A$ policeman blows a whistle of frequency $400 \ Hz$. $A$ car driver is approaching the policeman. The speed of the car is $54 \ km/h$. The change in frequency experienced by the driver,when the driver approaches the policeman and after he crosses the policeman,is ... $Hz$ [Velocity of sound is $350 \ m/s$]

This question has Statement $1$ and Statement $2$. Of the four choices given after the Statements,choose the one that best describes the two Statements.
Statement $1$: Bats emitting ultrasonic waves can detect the location of a prey by hearing the waves reflected from it.
Statement $2$: When the source and the detector are moving,the frequency of reflected waves is changed.

Statement $-1$: Due to the motion of the listener,the frequency of the sound waves (as received by the listener) emitted by a stationary source is affected.
Statement $-2$: Due to the motion of the source,the wavelength of the sound waves (emitted by the source) as received by a stationary listener is affected.
Statement $-3$: If the receiver and the source are both moving,the observed frequency must be different from the original frequency of the source.
Treat the motion of the source or listener as always along the line joining them for all the above cases.

$A$ train standing at the outer signal of a railway station blows a whistle of frequency $400\, Hz$ in still air. What is the frequency of the whistle for a platform observer when the train recedes from the platform with a speed of $10\, m/s$? (Speed of sound $= 340\, m/s$)

Match Column-$A$ with Column-$B$.

Column-$A$	Column-$B$
$(a)$ When source of sound and listener move towards wall.	$(i)$ Listener would hear a sound with same frequency as that of source.
$(b)$ When source of sound and listener move away from each other.	$(ii)$ Listener would hear a sound with frequency smaller than that of source.
	$(iii)$ Frequency of echo would be greater than that of source.

$A$ train approaching a railway platform with a speed of $20 \ ms^{-1}$ starts blowing the whistle. The speed of sound in air is $340 \ ms^{-1}$. If the frequency of the emitted sound from the whistle is $640 \ Hz$, the frequency of sound to a person standing on the platform will appear to be: (in $Hz$)