How Did Nubia Become a "Corridor" for Long-Distance Trade Before

How did Nubia become a "corridor" for long-distance trade before 3000 BCE?

A) Government policies carefully regulated and promoted trade.
B) Egypt built a river highway for the passage of goods.
C) Nubia had no natural resources of its own and so needed to import them.
D) It connected sub-Saharan Africa with North Africa.
E) Actually, Nubia never did participate in the African trade networks, preferring isolationism.