Select the statement that best summarizes the central idea of the article.

Electromagnets are useful in many situations because they can be turned on and off.
Motors change mechanical energy to electrical energy.
Electromagnets work whether or not electricity is passing through the wire.
Electromagnets are useful around the home but not in industry.

Electromagnets are useful in many situations because they can be turned on and off.

Radon-222 or rn-222 has a half-life of 3.82 days. to determine the fraction of an original sample of rn-222 that remains unchanged after 7.646 days, we calculate the amount of rn-222 remaining after 7.646 days and divide it with the original amount. using 1 gram as a basis and using half-life calculation methods, we can determine that  0.249 or 0.25  grams of rn-222 will remain, and thus the fraction remaining is 0.25 or 1/4.

It starts to rust i think