It's gravity I believe? I'm not entirely sure.
Letter B
without a medium, there is nothing to compress, hence, no wave. A fast- medium like a gas (air) is easy to compress and allows waves to move through it easily. a slow medium, like a liquid, is still pretty fast, but not as fast as air.
Answer:
The time rate of change of flux is

Explanation:
Given :
Current
A
Area of plate

Plate separation
m
(A)
First find the capacitance of capacitor,

Where 

F
But 
Where 


Now differentiate above equation wrt. time,



Therefore, the time rate of change of flux is

by the wind and air flow in the wind