Which best describes the way historians consider letters and journals

Historians consider letters and journals written by certain historical subjects to be absolutely crucial, since they represent an unaltered and oftentimes revealing ...

Journals and letters are primary sources of the individuals writing them. For example, Anne Frank's diary is a primary source explaining what it was like to live as a Jew during the holocaust. Primary sources are usually valued over secondary sources. A secondary source would be like someone writing a book about Anne Frank's journal. The person writing that book did not witness the events firsthand, but is drawing conclusions based on a primary source.