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