Current Research into Neurotransmitter Systems Has Produced the "Permissive Hypothesis

Current research into neurotransmitter systems has produced the "permissive hypothesis." What does this hypothesis state?

A) The norepinephrine system regulates serotonin levels; if norepinephrine is low, depression will occur.
B) The absolute levels of neurotransmitters are more significant in mood regulation than the overall balance of the various neurotransmitters.
C) Low levels of serotonin are sufficient to explain the etiology of mood disorders.
D) When serotonin levels are low, other neurotransmitter systems become dysregulated and contribute to mood irregularities.