The barriers to Southern transportation were the lack of railroads and the lack of decent roadways towards the Southern parts. Only the waterways were the alternative routes towards the South. They were not as developed compared to the Northern parts.
The Sassanian Empire controlled what is currently the Persian region and the Middle East, including at extent places like Egypt, Judea, and up to the Indus River. The Sassanian Empire and the Byzantine had a very turmoilous relationship resulting in multiple wars in the Irano-Byzantine wars from 282 to c. 628, which left both empires crippled.
the answer d im sure beacuase i put C like that person said and its wrong