Economists who study macroeconomics are concerned with all of the following except
a. the gross domestic product (gdp)
b, the business cycle
c, the purchases of single individual
d. assessing economic trends and practices of the aggregate

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: