Which describes a major challenge facing the builders of the transcontinental railroad?

a. bridging the rio grande river

b. spanning the grand canyon

c. tunneling under lake erie

d. crossing the sierra nevada mountains

Crossing the Sierra Nevada Mountains would be your answer. D.

It developed as a reaction to the impact of european colonialism in africa on peoples of african descent. in the  mid-20th century, activists in africa adopted pan-africanism as a rallying cry for independence from colonial rule. some african pan-africanists sought to unite the continent as one independenthope it was useful