Which of the following are transferred from one place to another place by the waves?

$A$ person fires a gun standing at a distance of $55 \, m$ from a wall. If the speed of sound is $330 \, m s^{-1}$,find the time taken for an echo to be heard. (in $s$)

In the given curve, half the wavelength is:

If the density of a medium through which sound is propagating is minimum,then this point is called:

$A$ stretched wire $0.5\, m$ long is made to vibrate in two different modes as shown in diagrams $(A)$ and $(B)$ given below:
$(i)$ If the wavelength of the wave produced in mode $(A)$ is $1\, m$,what is the wavelength of the wave produced in mode $(B)$ of the following diagram?
$(ii)$ In which case is the note produced louder? Give a reason for your answer.
$(iii)$ In which case is the pitch of the note produced higher? Give a reason for your answer.

Two astronauts cannot hear each other on the moon. Why?