The diagram shows the set-up for an electric doorbell. At bottom, a doorbell. Starting from midright, a wire attached to a contact screw runs to a power source and then to a bell and then to coil around both arms of a U shaped iron core. The coils plus core is labeled electromagnet with the lower arm of the core labeled N and the upper arm labeled S. The wire emerges from the coils to attach to a steel bar at its top end. The bottom end of the steel bar has a short piece of wire leading to a striker. Which best explains why the bell would not ring in this set-up? The circuit is open. The electromagnet has an iron core. The circuit is closed. The electromagnet is made of steel.