What can be concluded from the research on the effects of MDMA on the brain?

a. A few doses of MDMA are clearly harmful and should be avoided.

b. Studies have confirmed damage to dopaminergic but not serotonergic systems in the brains of animals exposed to heavy doses.

c. Human data from heavy users suggest that MDMA does cause serotonin abnormalities in the brain.

d. Animal studies typically show smaller MDMA effects than human studies.

e. Cognitive deficits in MDMA users are rare.