What physical feature of sub-Saharan makes transportation within the area most difficult

The Hot weather and the geography

Explanation:

Most parts of Africa are deserts and it makes it hard for a vehicle that is not offroad to use and drive to the destination.

answer; i believe its called ; checks   and balances;

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.