Many of the people in sub-saharan africa speak more than one language because

children must learn two languages to graduate from high school
most of the people of sub-saharan africa are linguistically talented
official business is conducted in a language other than the local language
the people of the region frequently travel between the world's continents

C. Official business is conducted in a language other than the local language. 

Many people in Sub-Saharan Africa grow up speaking the common dialect of their household or village, in which there are thousands. As they look for a career though, most businesses operate in either French or English. Thus, most Sub-Saharan Africans use their common dialect with their family and their business language in their workplace.