An electromagnet is built using a 9-volt battery with conducting wire connected to a switch and coiled around an iron nail. A pile of metal paper clips is moved close to the iron nail, yet no attraction is apparent between the electromagnet and the paper clips. What is the most likely conclusion to explain this result? A. The switch is turned off.
B. The switch is turned on.
C. The paper clips are made of a nonmagnetic metal.
D. There are not enough coils of wire around the iron nail.

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