What is the theory suggested as a possible explanation for the phenomenon in the following passage? "Many people believe that drinking a warm glass of milk at bedtime helps them fall asleep. The reason must be the tryptophan in the milk. Trypothphan is an essential amino acid taht is converted into serotonin. Serotinin affects sleep patterns and the perception of pain."

A. the tryptophan in milk increases serotonin levels, which helps people fall asleep

B. All amino acids help you sleep

C. drinking a warm glass of milk at bedtime helps people fall asleep

D. Trytophan is an amino acid