Which of the following is NOT a method of demagnetizing an object?

Placing an object in a strong magnetic field is not a method of demagnetizing it; in fact, it is a method that can potentially magnetize the object further or retain its magnetization, depending on the strength and orientation of the field. When an object is exposed to a strong magnetic field, the magnetic domains within the material can align more closely with the external field, reinforcing its magnetism.

In contrast, connecting the object to an alternating current line can demagnetize it by creating a varying magnetic field that disrupts the alignment of the magnetic domains. Heating the object can also demagnetize it, as high temperatures can cause thermal agitation, which disrupts the orderly alignment of the magnetic domains. Striking the object, such as with a hammer, can also be effective in demagnetizing, as mechanical shock can displace the alignment of these domains.

Therefore, option C correctly identifies a method that does not contribute to demagnetizing an object, while the other methods listed are effective approaches for that purpose.