In an Experiment, It Was Found That Newborn Rats Who

In an experiment, it was found that newborn rats who were genetically predisposed to be calm and stress-resistant grew upto be nervous, high-strung, and easily stressed adult rats when raised by genetically unrelated, inattentive mothers. The explanation for these results was that:

A) the rats' DNA changed because of their upbringing.
B) epigenetic changes had taken place that changed their brain chemistry and literally "reprogrammed" their future behavior.
C) severe genetic mutations had taken place from one generation to the next.
D) the rats' DNA was damaged by environmental factors, such as exposure to ultraviolet light, radiation, or chemical toxins.