What was the result of the expansion of slavery from 1800 to 1850

In the period between 1800 and 1850 America experienced a marked change in national unity. Territorial expansion had a large effect on national unity in this period. Territorial expansion boosted national unity between 1800 and 1823, but it degraded national unity between 1824 and 1850.

Explanation:because

option d is correct. corroborated sources of historical evidence   is necessary to validate a claim about an event in history.

historians tend to use written sources, but audio and visual materials as well as artifacts are important objects that supply information to modern historians.

historical sources are usually divided into two categories: primary and secondary sources. depending on the historian's intent, some sources shift their designation. determining what type of sources toimplement, and the level of credibility and reliability of those sources, is a significant step in critical thinking for the historian.

explanation:

he sent troops to protect the students as they entered the school.