Which Finding Provides Evidence That Opioid Drugs Enhance the Brain

Which finding provides evidence that opioid drugs enhance the brain reward system in animal models?

A) Administration of opioids lowers the electrical current threshold for self-stimulation.
B) Administration of opioids raises the electrical current threshold for self-stimulation.
C) Animals self-administer opioids at increasing levels over time.
D) Animals self-administer opioids at decreasing levels over time.