help pls!!!!!!!! Read the map.

A map titled Physical Features of East Asia. A key shows Lowlands in green, Highlands in yellow, and mountains with brown triangles. Large areas of yellow span across northern China and Mongolia, west toward Europe, east toward the rivers. Green areas run along the eastern and southern coastline and most of India.

What physical feature had the greatest effect on China’s contact with other countries?

Vast, flat deserts made northward travel easier.
Extensive highlands made westward travel difficult.
River systems helped travel between north and south.
High, dangerous mountains blocked southward travel.