Define tariff
Define quota
Define embargo

A tariff is a tax imposed by one country on the goods and services imported from another country.

Quota, a fixed share of something that a person or group is entitled to receive or is bound to contribute.

Embargo, an official ban on trade or other commercial activity with a particular country.

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.