What was not one of the ways in which the north benefited from slavery?

this is has missing info..

it is due to the halo effect as the impression of the boy created earlier.on the mind of the principle affected his decision

answer:

d. because dr. laguardia selected a widely used measure, his study generalizes to has strong external validity. external validity has limited external validity. other applications of this measure.

the widely used measure that he applied to his experiment was the benton facial recognition test. it employed on to football players and as result, only players who suffered brain concussions did not pas this test. and although it has an external validity, it is limited because it isn't general to apply to other population.