The correct answer is A. Exploration was limited because there were no rivers on which to travel Into the Interior.
Explanation:
In the excerpt, the author explains in the map of South Africa there are long rivers; however, these rivers are dried or completely dried most of the time, this is explained in "you find either a waterless bed or a mere line of green and perhaps unsavory pools."
Moreover, this feature of rivers makes them useless in regards to transportation, which caused the interior area of the country to be unexplored for a long time. This is explained in "People could not penetrate it by following waterways". Also due to this, explorations occurred by land but this limited the exploration. According to this, one effect of rivers was "Exploration was limited because there were no rivers on which to travel Into the Interior."
