Which of the following items could serve as a secondary source for a historian studying early chinese history?

a. a bronze plow discovered in the ruins of an ancient chinese city
b. a chart created in ancient china to track a city's growing population
c. an essay about china's first emperor written by a university professor
d. an announcement written for an early chinese emperor to deliver to his subjects

C. An essay about China's first emperor written by a university professor

Explanation:

A university professor's research and analysis is more credible than;  

A bronze plow discovered in the ruins of an ancient Chinese city

A chart created in ancient China to track a city's growing population

An announcement written for an early Chinese emperor to deliver to his subjects

mass production/ assembly line production

the answer here would be