What landform blocked contacts between the people in northern china and those in southern china?

Huai River Qin Mountains Line landform blocked contacts between the people in northern China and those in southern China.

Huai River Mountains Line is the landform that is setting out as the dividing line to be located as between the northern and southern China.  It is located at the region of somehow midway between the Yellow River and Yangtze.

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.