How do our ears permit us to receive sound?

An astronaut cannot hear his companion on the surface of the Moon because

$(a)$ The sound of an explosion on the surface of a lake is heard by a boatman $100 \ m$ away and a diver $100 \ m$ below the point of explosion. Of the two persons mentioned (boatman or diver),who would hear the sound first and why?
$(b)$ Calculate the wavelength of a sound wave whose frequency is $220 \ Hz$ and speed is $440 \ m \ s^{-1}$ in a given medium.

$A$ girl is sitting in the middle of a park of dimension $12 \,m \times 12 \,m$. On the left side of it there is a building adjoining the park and on the right side of the park,there is a road adjoining the park. $A$ sound is produced on the road by a cracker. Is it possible for the girl to hear the echo of this sound? Explain your answer.

Draw a curve showing density or pressure variations with respect to distance for a disturbance produced by sound. Mark the position of compression and rarefaction on this curve. Also,define wavelengths and time-period using this curve.

State whether the following statement is true or false: Sound and light travel in a medium in the form of crests and troughs and rarefactions and compressions respectively.