From the 1920s into the 1960s, American psychologists emphasized the study of . However, with the support of new brain imaging technology, American psychologists have returned to psychology’s roots by emphasizing the study of .

mental life; observable behavior

Explanation: Mental life has a lot to do with how psychologists focus more on the physiological research in the 1920s and 1960s, but as times goes by, they emphasize more on observable behavior. Because both mental life and observable behavior work together for a good psychological result.

answer: proteins

explanation: complement is   system of circulate in the blood in a inactive form

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