What was the contribution of the German psychiatrist Emil Kraepelin to the study of abnormal psychology? A.
He introduced the surgical procedure called lobotomy.
B.
He was the first to classify mental disorders.
C.
He founded the National Committee for Mental Hygiene.
D.
He presented the psychoanalytical theory of personality.

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Okay, biology isn't exactly my strongest subject (it never has been), but i'm pretty sure the answer is nucleotides. dna is made up of nucleotides. each nucleotide contains a phosphate group, a sugar group, and a nitrogen base. there are four different nitrogen bases: adenine (a), thymine (t), guanine (g), and cytosine (c). a always pairs with t, and g always pairs with c.