An electromagnet is different from a permanent magnet because an electromagnet

is magnetic
can push and pull
can attract some metals
can turn on and off

can turn on and off because it is active through an electric current and when the current is gone it no longer works . electromagnets, - devices that create a magnetic field through the application of electricity. Wrecking yards employ extremely powerful electromagnets to move heavy pieces of scrap metal or even entire cars from one place to another. Your favorite band uses electromagnets to amplify the sound coming out of its speakers. And when someone rings your doorbell, a tiny electromagnet pulls a metal clapper against a bell.

A.(nh4)(2co3)is the correct answer 

The answer is b because a,c,and d are opinons