Why did the Mongols repeatedly fail to conquer the Sung dynasty before Kublai Khan’s conquest?

A. The Sung army had trebuchets.
B. The Sung army had a blockade.
C. The Sung army had battering rams.
D. The Sung army had greater numbers.

first, the romans stayed isolated because they had to defend their own territories from neighboring cities that were harassing them, like the etruscans. second, once the romans managed to defeat their neighbors, and started to expand their territory, they started to build roads and bridges. finally, now that the romans have covered their territory with roads functioning as highways, they could mobilize their legions faster to assemble larger military units in a specific place.

the correct answer on edgen is:

(c.) prevent the spread of communism.

i just took the test.