Which statement best describes the populations of countries in the sahel?

The Sahel part of Africa includes (from west to east) parts of northern Senegal, southern Mauritania, central Mali, northern Burkina Faso, the extreme south of Algeria, Niger, the extreme north of Nigeria, central Chad, central and southern Sudan, the extreme north of South Sudan, Eritrea, Cameroon, Central African Republic and the extreme north of Ethiopia  

Traditionally, most of the people in the Sahel have been semi-nomads, farming and raising livestock in a system of transhumance, which is probably the most sustainable way of utilizing the Sahel. The difference between the dry North with higher levels of soil nutrients and the wetter South with more vegetation, is utilized by having the herds graze on high quality feed in the North during the wet season, and trek several hundred kilometers to the South to graze on more abundant, but less nutritious feed during the dry period

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.