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.

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ans.

heterothermic organisms include those organisms that can switch between homeothermic (having body temperature higher than environment) and poikilothermic (having variation in internal body temperature) strategies.

some migratory birds show heterothermy as they decrease their metabolic rate during cold nights to keep their body temperature low. drop in metabolic rate allows increase in production efficiency of birds that reduces the internal body temperature to make it equal to external cold temperature during cold nights.

thus, the correct answer is 'birds allow their temperature drop in order to increase their production efficiency.'

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