Question

Which lists a geographical feature that has kept the Balkan Peninsula isolated, and a second feature that helps overcome that isolation?

Surrounded by three seas/mountainous terrain

Mountainous terrain/surrounded by three seas

Alpine mountains/network of navigable rivers

Network of non-navigable rivers/flat, inland plains​

Answer 1

Mountainous terrain and surrounded by three seas. Mountains always have been natural barriers, just as India is separated from China by a mountain. The seas provide protection as well because boats would take a longer time to cross them and storms, hurricanes, large waves, etc could stir up and wreck many invaders.