What are the characteristics of a
unit?

Two characteristics of a unit are:

It should be of convenient size.

It must be universally accepted i.e. its value must remain the same at all places and at all times.

true is the correct answer

hope i

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me