Read this excerpt from a letter by Civil War soldier Sullivan Ballou:

I have no misgivings about, or lack of confidence in the cause in which I am engaged, and my courage does not halt or falter. I know how strongly American Civilization now leans on the triumph of the Government and how great a debt we owe to those who went before us through the blood and sufferings of the Revolution. And I am willing—perfectly willing—to lay down all my joys in this life, to help maintain this Government, and to pay that debt.

Why is this letter an important primary source document?
A.It explains the cause of the American Revolution.
B.It explains what he means by American Civilization.
C.It gives insight into what the soldiers believed.
D.It shows where the key battles occurred.

A primary source refers to as photographs, texts, records, letter,etc. that were created during an event or shortly after it, usually by someone who experienced the event and therefore are direct account on the events. But besides this, one of the most important features of primary sources is that they provide the perspective of people about the event that helps researches and other experts to understand an event in a holistic way.

In the case presented, there is an excerpt of a letter wrote by a soldier during the Civil War that tells the perspective of the soldier about the event as he says he feels confident and courageous and besides this believes all the American Civilization is waiting for the triumph. This means, this source can be classified as an important primary source not only because it was written by someone that directly experience the event but because it provides the perspective of a soldier that was probably shared by many others, which is useful information to understand the civil war. Therefore, this letter is an important primary source document because "it gives insight into what the soldiers believed".