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It can be seen from the observation diagram that when the magnetic object reaches directly below the electromagnet AB and the switch S is immediately closed, it can be seen from Ampere's Law that the B end of the electromagnet is the N pole.

Because the top of the magnetic object is N-pole, when the magnetic object passes directly below the electromagnet, it will be subjected to a great repulsive force, which will increase the pressure of the magnetic object on the desktop, and the friction will also increase under the condition that the roughness of the contact surface remains unchanged.

The magnetic object moves in a straight line at a uniform speed on the horizontal plane, and the tension is constant, which is less than the friction force, so it slows down immediately.

So choose C.