Suppose that a certain atom possesses only four distinct energy levels. Assuming that all transitions between levels are possible, how many spectral lines will this atom exhibit? 
Definitions:
Threshold
The minimum level of stimulus intensity required to trigger a perceptible response or reaction.
Minimum Level
The lowest degree or amount of something that is considered acceptable or sufficient.
Antagonist
A molecule that inhibits or blocks a neurotransmitter’s action.
Neurotransmitter Receptor
A protein molecule located on the surface of cells, which binds to specific neurotransmitters and triggers a response in the cell.
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