The major conclusion to be drawn from Rosenzweig's study of the brains of rats raised in an enriched environment is that:

a. early experience shapes the way in which the brain develops
b. genes exert their greatest influence in guiding neural development during the prenatal period
c. too much "privilege" can cause the development of selfishness that cannot be undone through later training
d. neural development is for the most part determined by genes, and especially by those carried on the sex chromosomes

a