The answer is A. The settlers killed of most of the Native Americans buffalo with they hunted for food.
In the 1850s, new types of machinery being produced increasingly needed oil for lubrication. And the main sources for oil at the time, whaling and collecting oil from seeps, simply couldn't meet the demand. Someone had to find a way to reach into the ground and extract the oil.
There are five main causes to the civil war, Slavery, Economy, Federal versus State rights, Abraham Lincoln becoming president and the Battle of Fort Sumter.
The homestead act freed African Americans to settle in Great Plains.