Answer: the decision by the United States government to prevent additional settlement
Explanation:
What was meant by the "closing" of the Western frontier was that the United States government wanted to prevent additional settlement.
This decision was as a result of the government's plan to promote the westward expansion after the Civil War, but however, they had to close the Western frontier to prevent additional settlers.
By Compare the artifact to other things found at the sites they determine its relative age.
Compare the artifact to other things found at the same place.
<u>Explanation:</u>
Over the period of time, the artifacts which are lying under the ground gets covered by the layers and layers of soil and the archaeologists can determine the age of the artifacts by relative technique where the artifacts dug out from the same sites are compared and analysed and then it is approximately given the relative age.
Though it is not a very reliable method as the artifacts gets shifts from place sometimes and may go deeper under the soil or come closer to the surface. Therefore, Absolute method was discovered and is used.
It would be C) it decreased the amount of pirates and thieves
