The sahara served as a(n) the rest of the world.
a. oasis/uniting africa with
b. barrier/isolating africa from
c. avenue for trade/connecting africa with
d. means for isolation/barricading africa from

Sahara is a desert and deserts are hard to cross. Sahara is especially bad and potentially deadly: it's very dry and there is no water to be found for miles.

Therefore it was a barrier and it has isolated South Africa from European influence for a long time. (until the time of European explorations)

Developed countries will see a decrease in natural resources, because their population will decrease.