The frequency of a whistle of an engine is $600 \, Hz$ and it is moving with a speed of $30 \, m/s$ towards a stationary observer. The apparent frequency heard by the observer will be .... $Hz$ (velocity of sound $= 330 \, m/s$).

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.

$A$ bat is flitting about in a cave,navigating via ultrasonic beeps. Assume that the sound emission frequency of the bat is $40\; kHz$. During one fast swoop directly toward a flat wall surface,the bat is moving at $0.03$ times the speed of sound in air. What frequency (in $kHz$) does the bat hear reflected off the wall?

$A$ man sitting in a moving train hears the whistle of the engine. The frequency of the whistle is $600 \ Hz$.

$A$ source and an observer move away from each other with the same velocity of $10 \,ms^{-1}$ with respect to the ground. If the observer finds the frequency of sound coming from the source as $1980 \,Hz$, then the actual frequency of the source is (speed of sound in air $= 340 \,ms^{-1}$). (in $\,Hz$)

$A$ source of sound $S$ is moving with a velocity $50 \ m/s$ towards a stationary observer. The observer measures the frequency of the source as $1000 \ Hz$. What will be the apparent frequency of the source when it is moving away from the observer after crossing him? The velocity of sound in the medium is $350 \ m/s$.