Concept:Low river traffic in Africa is mainly due to natural barriers that hinder navigation.
Explanation:Most African rivers flow across plateaus and hard rock beds.
This creates rapids, waterfalls, and cataracts along the river course.
These features break the river into sections and block the movement of boats and ships.
Continuous navigation therefore becomes difficult, so fewer vessels use the rivers.
Other factors are unimportant here when compared with these physical obstacles.
Answer:A. Presence of rapids and waterfalls