In 1976, a historian is writing a book analyzing the causes of the American Revolution, which was fought 200 years earlier. Which of the following sources would be a primary source he could consult?

A. A book on the revolution written by a famous American historian in the late 1800s

B. A well-known American history textbook used in schools throughout the country

C. A letter from an American soldier describing why he supported the American Revolution

D. An award-winning documentary describing competing theories on the revolution's causes