A group of rats was exposed to the uncontrollable stress associated with being forced to swim for 15 minutes in cold water from which they cannot escape. What did researchers notice when they observed these animals?

a. Most rats were immune to the effects of the stress.
b. The rats' depression was reduced by the high level of physical activity.
c. The stress caused a form of brain damage that first led to motor retardation and then death.
d. The rats that exhibited symptoms similar to depression had altered neurotransmitter levels.

Answer: d.