There were many. Timber, Silver, Gold, Iron ore, coke, steel (these last 3 actually go together and the last one is derived from iron ore), the raw materials for a chemical industry, wheat (barley oats, cattle, livestock these last I consider natural resources. Your teacher may not), grass lands (for ranches). That ought to get you started.
Because of the discovery of gold in San Francisco in 1848, many people began moving west to search for wealth and fame. The United States was discovering many things underground...gold, silver, and oil. These were some of the natural resources the United States had at the end of the 1800s.
The US thought they could wear down the Vietcong with high casualties but the Vietcong saw the battle as a fight for their lives so they didn't wear down easily. Hope this helps!
The correct answer should be the third one you listed. The "current drag" then is a device that is used in order to measure the sub-surface currents. This device and the measurements it provides are extremely important for the field of oceanography which studies the aspects of the oceans both the physical and the biological ones.
