HI! I NEED HELP ASAP!!
Why are electromagnets used in metal scrap yards?

A. They are temporary magnets, so they can be turned on and off.

B. They are very powerful permanent magnets.

C. They are weak permanent magnets.

D. They are temporary magnets, so eventually the metal just falls off.

Amichelson interferometer operating at a 600 nm wavelength has a 3.02-cm-long glass cell in one arm. to begin, the air is pumped out of the cell and mirror m2 is adjusted to produce a bright spot at the center of the interference pattern. then a valve is opened and air is slowly admitted into the cell. the index of refraction of air at 1.00 atm pressure is 1.00028.how many bright-dark-bright fringe shifts are observed as the cell fills with air?