Definition: You are given a question on high school psychology. You are also given 4 answer options (associated with "A", "B", "C", "D"), out of which only one is correct. You need to answer the question by selecting the correct option. You should only answer with the choice letter, not the whole answer.
Input: The instrument that has traditionally been used in sleep research to assess activity in the brain is called the
(A)signal detection device (SDD) (B)computerized axial tomography (CAT) (C)magnetic resonance imaging (MRI) (D)electroencephalograph (EEG)

D