Read the excerpt from “Tools of the Spymaster." The numbers that Major Tallmadge assigned to members of the Culper Ring were from a secret writing system he invented. He substituted digits for words that would be used in messages. "Long Island," for example, was 728, "arms" was 7, and "city" was 88. There was a number for each month, such as 341 for "January." He made four copies of his codes. He kept one and gave the others to Woodhull, Townsend, and General Washington. For words that did not have a number code, Tallmadge gave his agents a cipher. In a cipher, each letter in a message is replaced by another letter or a number.

The author’s purpose in this excerpt is to

A)teach readers about a code system used during the American Revolution.
B)persuade readers that ciphers are the greatest American invention.
C)argue that Major Tallmadge made a mistake in sharing copies of his codes.
D)compare the American codes for secret letters to the British codes.