When sound waves travel from air to water,which of the following remains constant?

The depth of the sea is measured by means of an echo. If the time between the moment the sound is produced and received is $3.52 \,s$, then the depth of the sea is (The compressibility of sea water is $4.84 \times 10^{-10} \,m^2 \,N^{-1}$ and its density is $1024 \,kg \,m^{-3}$): (in $km$)

The velocity of sound in air is doubled when the temperature is raised from $0^{\circ} C$ to $\alpha^{\circ} C$. The value of $\alpha$ is . . . . . . .

$A$ man,standing between two cliffs,claps his hands and starts hearing a series of echoes at intervals of one second. If the speed of sound in air is $340 \, ms^{-1}$,the distance between the cliffs is .... $m$

Find the wavelength of a sound wave of frequency $4.2 \text{ MHz}$ travelling with a speed of $1.7 \text{ km/s}$.

The minimum distance of the reflector surface from the source to hear the echo of sound is ..... $m$.