Many doorbells work in the way pictured below.  an electromagnet, which is made up of a coil of wire wrapped around a metal core, causes a metal beater bar to rapidly move up and down.  this causes the beater at the end of the bar to strike a bell repeatedly. 

how does the electromagnet cause the beater bar to move?
  a. 
an electric current flows through the bell, which electrically charges the beater bar and the electromagnet, causing it to vibrate.
  b. 
an electric current flows through the electromagnet, into the beater bar, and into the bell, causing it to ring.
  c. 
the electromagnet repeatedly bangs into the beater bar, causing the beater to repeatedly strike the bell.
  d. 
an electric current flows through the electromagnet's wire coil and generates a magnetic field, which produces a force on the beater bar.