Highlight the big ideas and ideas that are repeated to find the central idea.

The British often used a "mask" to keep their messages secret. Hidden within what looked like an ordinary letter was the secret message that was only revealed when the reader applied the mask. The writer might send the letter by one rider going by one route while another rider carried the mask on another route. Such letters had an extra advantage: The writer could include false information in case Americans intercepted the letter.

—George Washington, Spymaster,
Thomas B. Allen

Which statement best states the central idea?

A mask was a useful way to send messages.
Letter writers could travel via several routes.
Writers could include false information.
A mask was needed to read a message.