A_ property is a characteristic of st substance that can be observed and does not change identity of a substance

physical

Explanation:

Physical property is the property of a substance which does not involve rearrangement of atoms and deals only with size, shape and state of the substance. Thus the identity of the substance does not change. Example: Melting point, hardness

Chemical property is the property of a substance which involves rearrangement of atoms and thus new substances are formed. Thus the identity of the substance change. Example: Oxidation , Precipitation

the answer is b

explanation:

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