(20 1 1? In order to enhance their sense of security, several residents in a dormitory building installed security doors on the stairs of the unit, as shown in the figure.

As can be seen from the figure, the current flows in from the right side. According to the right-hand spiral law, the left end of the electromagnet is N pole and the right end is S pole. Therefore, ① is correct;

Because each user has to control the door lock independently, that is, a switch can open the door lock, so only parallel circuits can complete this function; If it is in series, all switches should be turned off to control the door lock; Therefore, 3 is correct;

So choose a.