Which best contrasts electric doorbells and audio speakers?

doorbells involve rapid changes in the direction of current, and speakers do not.
speakers involve rapid changes in the direction of current, and doorbells do not.
doorbells require closed circuits in order to work, and speakers do not.
speakers require closed circuits in order to work, and doorbells do not.

The only true statement among the choices is:
 
                       Speakers involve rapid changes in the
                       direction of current, and doorbells do not.

The other three statements are false.

Answer;

Speakers involve rapid changes in the direction of current, and doorbells do not.

Explanation;Speakers are made of a permanent magnet surrounding an electromagnet which is attached to the loudspeaker membrane or cone. By varying the electric current through the wires around the electromagnet, the electromagnet and the speaker cone can move  back and forth. If the variation of the electric current is at the same frequencies of sound waves, the resulting vibration of the speaker cone will create sound waves, including that from voice and music.Electric door bells use electromagnets, which is a temporary magnet whose magnetism can be varied by changing the amount of current. pressing the switch closes the contact and thus completing the circuit and the gong hits the bell breaking the circuit again. Both the speakers and the doorbells use the concept of electromagnetism however, they differ in that the speakers involve rapid changes in the direction of current, and doorbells do not, instead, it relies on completing and breaking the circuit.

Iassume the answer is 75 feet. inwards to outwards rather than outwards to inwards. probably be 75.hachures are stokes drawn in direction of the steepest slope. a series of short parallel lines drawn on a map to indicate topographic relief. it is actually called hachures on contour lines. the small marks in the center of the feature are called hachures.observe the picture attached