Doppler's effect will not be applicable when the velocity of sound source is

$A$ train is moving towards a stationary observer (at $t = 0$) with a constant velocity of $20 \ m/s$ and after some time it crosses the observer. Which of the following curves best represents the frequency $f$ received by the observer as a function of time?

$A$ train moves towards a stationary observer with a speed of $34 \ m/s$. The train sounds a whistle and its frequency registered by the observer is $f_1$. If the train's speed is reduced to $17 \ m/s$,the frequency registered is $f_2$. If the speed of sound is $340 \ m/s$,then the ratio $f_1/f_2$ is:

Two cars $A$ and $B$ are moving in the same direction with speeds $36 \, km/hr$ and $54 \, km/hr$ respectively. Car $B$ is ahead of $A$. If $A$ sounds a horn of frequency $1000 \, Hz$ and the speed of sound in air is $340 \, m/s$,the frequency of sound received by the driver of car $B$ is ..... $Hz$.

Suppose that the speed of sound in air at a given temperature is $400 \ m/s$. An engine blows a whistle at $1200 \ Hz$ frequency. It is approaching an observer at the speed of $100 \ m/s$. What is the apparent frequency as heard by the observer in $Hz$?

Two sources $A$ and $B$ are producing notes of frequency $680 \,Hz$. $A$ listener moves from $A$ to $B$ with a constant velocity $v$. If the speed of sound in air is $340 \,ms^{-1}$, the value of $v$ so that he hears $10$ beats per second is: (in $\,ms^{-1}$)