Why was the kingdom of mali more successful than gahna

The kingdom of mali is more successful than the kingdom of Ghana because of the Niger river which provides a more reliable economy to the kingdom of Mali. Mali was located in an area along the upper Niger Riverwhere agricultural activities flourished. Camels and donkeys remained a passe' and old which was still used by Ghana. 

Answer (d) may increase or decrease porosity through dissolution or deposition;