I believe the answer would be C, burning fossil fuels. Oil and gas is made up of the remains of microscopic plankton. Over millions of years the remains become the carbon-rich coal, oil and gas we can use as fuel. When fossil fuels are burned they release carbon dioxide into the atmosphere, thus contributes to global warming.
In 1828, Jackson was elected president. He declared that the only hope for the Southeastern tribes' survival would be for them to give up all their land and move west of the Mississippi River. ... The U.S. government promised to compensate the tribes for the property they would have to abandon.
The US forces passing the 38th parallel would be your answer.