Neuronal Regeneration in the Adult Mammalian CNS Does Not Occur

Neuronal regeneration in the adult mammalian CNS does not occur as easily as it does in the immature mammalian CNS because

A) there are more excitatory factors/neurotransmitters in the immature CNS than in the adult.
B) there are more inhibitory factors/neurotransmitters in the immature CNS than in the adult.
C) adult CNS axons have more myelination than that of immature CNS.
D) adult CNS neurons are fully differentiated, compared to that of immature CNS.
E) adult CNS is less permissive overall than is the immature CNS.