For most materials, however, the resistance does depend on the current. Resistance, classically, is caused by collisions between the charges carrying the current and other charges and atoms. For metallic objects, as more voltage is placed across the object driving more current, the object becomes hotter, which increases the vibrations of the atoms and, in turn, increases these collisions and, hence, the resistance of the object. Of the plots below, then, which plot might correspond to the current-voltage characteristics of a metal then.