How Did the Spread of Islam in East Africa Tend

How did the spread of Islam in East Africa tend to differ from the way it spread in Central and West Africa?

A) East Africa was Islamized by overland routes,while West Africa was converted by sea routes.
B) East Africa was Islamized by sea routes,while West Africa was converted by overland routes.
C) West Africa was never Islamized as was East Africa.
D) East Africa was converted by force and West Africa by choice.