What geographic feature forms Vietnam's western border?

Mekong River

Red River

Alps

Annam Highlands

Mekong River

Explanation:

Vietnam's borders are areas covered by rivers, valleys and mountains. Most of the Western border, along with mountain ranges and dense forests, stretches the Mekong River and its delta. The interior is made up of valleys and hills with forests.

The most populated areas are those where agriculture is widespread.