Why is sahara considered a natural boundary? ?

  The Sahara desert is huge, pretty much the size of the United States, and it is one of the most hostile places on the planet. The desert is vast, very dry, the landscape occupied by sand dunes and eroded rocks, vegetation and animal life is scarce, and there's almost no water in it. Also a big problem are the sand storms that can last for days or even weeks.

  Because of all this hostile conditions, people have always avoided passing through it because they would be risking their lives, and the chances that they will be able to get through it safely and unharmed are extremely low.

answer; (d) melting of sea ice;

the proximity of water was an advantage to western europe and its culture was able to flourish as a result. trade and travel by sea were easier and safer in historical times.

1. there are no mountains to protect it from the sea

2. and its has wet cool summers and wet, mild winters.