Which of the following statements does not demonstrate the highly reinforcing effects of opiates?
a. Animals readily self-administer all known opiate agonist drugs.
b. Opiates are the most effective pain-reducing drugs available to physicians.
c. The chronic use of opiates often leads to tolerance and physical dependence.
d. Naloxone blocks the effects of opiate self-injection.
e. Opiate drugs have medicinal use but also great potential for abuse.