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

The speed of sound in oxygen $(O_2)$ at a certain temperature is $460 \,ms^{-1}$. The speed of sound in helium $(He)$ at the same temperature will be (assume both gases to be ideal): (in $\,ms^{-1}$)

The figure below shows pressure variation in two different sound waves in air with time at a given position. Both the figures are drawn to the same scale. Which of the following statements is true?

Regarding the speed of sound in a gas, match the following:

$A$. Temperature of gas is made $4$ times and pressure $2$ times	$P$. Speed becomes $2\sqrt{2}$ times
$B$. Only pressure is made $4$ times without change in temperature	$Q$. Speed becomes $2$ times
$C$. Only temperature is changed to $4$ times	$R$. Speed remains unchanged
$D$. Molecular mass of the gas is made $4$ times	$S$. Speed becomes half

The enclosed air inside a closed box of rigid walls has pressure $P_1$. The density of air inside this box is constant. On heating the gas, the pressure of the enclosed air is increased from $P_1$ to $P_2$. Now it is observed that the sound travels $1.4$ times faster than at pressure $P_1$. The ratio $P_2/P_1$ is:

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 $km$)