$A$ train is moving towards a stationary observer. Which of the following curves best represents the frequency $f$ received by the observer as a function of time $t$?

$A$ source of sound is travelling with a velocity of $40 \, km/h$ towards an observer and emits sound of frequency $2000 \, Hz$. If the velocity of sound is $1220 \, km/h$,what is the apparent frequency heard by the observer in $Hz$?

$A$ train approaching a railway crossing at a speed of $120 \ km/h$ sounds a whistle of frequency $576 \ Hz$,when it is $288 \ m$ away from the crossing. The frequency heard by an observer standing on the road perpendicular to the track at a distance of $384 \ m$ from the crossing is (Speed of sound in air $= 340 \ m/s$): (in $Hz$)

The frequency of the echo will be $.......Hz$ if a car blowing a whistle of frequency $320\,Hz$ is moving with a velocity of $36\,km/h$ towards a hill from which an echo is heard by the car driver. The velocity of sound in air is $330\,m/s$.

$A$ stationary source emits sound waves of frequency $500\, Hz$. Two observers moving along a line passing through the source detect sound to be of frequencies $480\, Hz$ and $530\, Hz$. Their respective speeds are,in $m\,s^{-1}$ (Given speed of sound $= 300\, m/s$)

Two men are walking along a horizontal straight line in the same direction. The man in front walks at a speed $1.0 \ m \ s^{-1}$ and the man behind walks at a speed $2.0 \ m \ s^{-1}$. $A$ third man is standing at a height $12 \ m$ above the same horizontal line such that all three men are in a vertical plane. The two walking men are blowing identical whistles which emit a sound of frequency $1430 \ Hz$. The speed of sound in air is $330 \ m \ s^{-1}$. At the instant when the moving men are $10 \ m$ apart,the stationary man is equidistant from them. The frequency of beats in $Hz$ heard by the stationary man at this instant is: