Which pre-assumptions are used for the Doppler effect?

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

$A$ sounding body of negligible dimension emitting a frequency of $150\, Hz$ is dropped from a height. During its fall under gravity,it passes near a balloon moving up with a constant velocity of $2\, m/s$ one second after it started to fall. The difference in the frequency observed by the man in the balloon just before and just after crossing the body will be: (Given: velocity of sound $= 300\, m/s$; $g = 10\, m/s^2$) (in $, Hz$)

$A$ railway engine whistling at a constant frequency moves with a constant speed. It goes past a stationary observer standing beside the railway track. The frequency $(n)$ of the sound heard by the observer is plotted against time $(t).$ Which of the following best represents the resulting curve?

$A$ source of sound $A$ emitting waves of frequency $1800\,Hz$ is falling towards the ground with a terminal speed $v.$ The observer $B$ on the ground directly beneath the source receives waves of frequency $2150\,Hz.$ The source $A$ receives waves,reflected from the ground,of frequency nearly ..... $Hz$ (Speed of sound $= 343\,m/s$)

Two trains $A$ and $B$ are moving with speeds $20 \ m/s$ and $30 \ m/s$ respectively in the same direction on the same straight track,with $B$ ahead of $A$. The engines are at the front ends. The engine of train $A$ blows a long whistle. Assume that the sound of the whistle is composed of components varying in frequency from $f_1=800 \ Hz$ to $f_2=1120 \ Hz$,as shown in the figure. The spread in the frequency (highest frequency - lowest frequency) is thus $320 \ Hz$. The speed of sound in still air is $340 \ m/s$.
$1.$ The speed of sound of the whistle is
$(A)$ $340 \ m/s$ for passengers in $A$ and $310 \ m/s$ for passengers in $B$
$(B)$ $360 \ m/s$ for passengers in $A$ and $310 \ m/s$ for passengers in $B$
$(C)$ $310 \ m/s$ for passengers in $A$ and $360 \ m/s$ for passengers in $B$
$(D)$ $340 \ m/s$ for passengers in both the trains
$2.$ The distribution of the sound intensity of the whistle as observed by the passengers in train $A$ is best represented by
$3.$ The spread of frequency as observed by the passengers in train $B$ is
$(A)$ $310 \ Hz$ $(B)$ $330 \ Hz$ $(C)$ $350 \ Hz$ $(D)$ $290 \ Hz$
Give the answer for question $1, 2$ and $3$.