20. Current research into neurotransmitter systems has produced the "permissive" hypothesis,which states that
(A) low levels of serotonin are sufficient to explain the etiology of mood disorders.
(B) the norepinephrine system regulates serotonin levels； if norepinephrine is low,depression will occur.
(C) when serotonin levels are low, other neurotransmitter systems become dysregulatedand contribute to mood irregularities.
(D) the absolute levels of neurotransmitters are more significant in mood regulation thanthe overall balance of the various neurotransmitters.