Located in South East Asia and approximately 10,000 miles away from the Continental United States the Vietnam climate and geographical plains made it hard to travel and transport supplies.
Much of Vietnam, especially the mountains is covered by dense jungle (The Embassy of the Socialist Republic of Vietnam in the United Kingdom) Has 2 major rivers used largely for transportation.
Southern Vietnam was bordered by Laos and Cambodia and Was divided by the US military into FOUR sections making communication and back up difficult. also, the United States at the time were not trained widely for jungle battle, and were extremely vulnerable for surprise attacks.
The answer is D. It provided a source of transportation for goods and people
