Current research into neurotransmitter systems has produced the "" hypothesis, which states that when serotonin levels are low, other neurotransmitter systems become dysregulated and contribute to mood irregularities.

permissive

Explanation:

Here is the complete question:.

Current research into neurotransmitter systems has produced the " " hypothesis, which states that when serotonin levels are low, other neurotransmitter systems become dysregulated and contribute to mood irregularities.

a. incompatibility

b. lock-and-key

c. permissive

d. pathoneurosis

amber has allowed scientists to study dna that has been preserved over millions of years.