What is the difference between a letter written by an eyewitness to an event
and an encyclopedia entry about the same event?

a. the letter is a primary source, while the encyclopedia entry is a
secondary source.

b. the letter is a periodical source, while the encyclopedia entry is a
secondary source.

c. the letter is a chronological source, while the encyclopedia entry
is a primary source.

d. the letter is a secondary source, while the encyclopedia entry is a
primary source.