Why Was Cognition Somewhat Ignored as a Topic of Study

Why was cognition somewhat ignored as a topic of study in psychology for many years

A)  It can't be studied in animals, so few researchers were interested. 
B)  The equipment that is necessary to image the brain was not developed yet. 
C)  Interest in psychodynamic theory dominated the field. 
D)  It is incredibly difficult to study thoughts in a controlled manner due to the unobservable nature of introspection.