The correct answer is - C. the Nile is too shallow.
The Nile is by no means a shallow river, it is more than able to sustain a river traffic with boats of small and medium size between it's delta on the north and the highlands on the south. There's another thing that is a problem, and that is the delta of the river. Nile's delta is pretty shallow and it doesn't allow free moving to be able between the river and the Mediterranean Sea. So people in Egypt just make what they can out of it, by using small boats they manage to travel through the river, or fish, or use it for tourist purposes, but that is limited only in the interior of the river.
