Read the excerpt from “Tools of the Spymaster." Washington, a spymaster with a sharp eye for details, gave exact instructions on how an agent should use invisible ink: "[H]e should occasionally write his information on the blank leaves of a pamphlet . . . a common pocket book, or on the blank leaves at each end of registers, almanacs, or any new publication or book of small value." He also told his agents that a "much better way" is to write a letter "with some mixture of family matters" and the secret message, which would be invisible, "between the lines and on the remaining part of the sheet."

What is the author’s main purpose in the excerpt?

to inform readers about George Washington’s stealth and creativity
to persuade readers that spying during wartime is heroic
to teach readers about the dangers of being a spy
to compare various methods of writing secret messages