Can mental exercise build "mental muscle"? In one study of this question, 12 littermate pairs of young male rats were used; one member of each pair, chosen at random, wa-

a) Put in an enriched environment with various mental challenges.
b) Placed in a standard environment without additional mental stimuli.
c) The cognitive abilities of both groups were assessed over time.
d) Results indicated a positive correlation between mental exercise and cognitive development.