Sub-saharan africa is studied as a region because
a.
most people speak the same language
b.
most people belong to the same ethnic group
c.
it is isolated from north africa by the sahara desert
d.
most people practice the same religion

C.it is isolated from North Africa by the Sahara Desert

Explanation:

Region can be divided by different ways, by the language they speak, by the religion they practice, by the country or type of government that they beling to, or because of their geographical barriers, in this case the Sahara Desert isolates the sub sharan region and it has certain characteristics because of that and that allows the scientists to study that region separetly.

the doctrine of nullification recommended that states living inside the union have the one-sided, inalienable ideal to void any law made by the national government that could be esteemed illegal.

the united states was shaped based on a general agreement among its individual states. hence, it is inferred that under the hypothesis of invalidation, since the states are the establishment of the union, they have the ability to referee and disprove unlawful laws.