Answer:
the anwser is d
Explanation:
you must have a north and south pole
Explanation:
We have,
Surface area, 
The current varies wrt time t as :

(a) At t = 2 seconds, electrical charge is given by :

(b) Current is given by :

Instantaneous current at t = 1 s is,

(c) Current is, 
Current density is given by electric current per unit area.

Therefore, it is the required explanation.
This would be known to be determined by its sound wave which would be the amplitude. The pitch of sound would actually be a little differenr in this matter. But know that we know and understand the sound wave of the pitch of sound, we know that this is done by amplitude.