Listen to a radio programme and read the statements. There is a mistake in each statement. Write the correct words or numbers in the box under the statement. Do not use more than 2 words!
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1. Sally Evans has worked for the American Secret Service for over 20 years.
2. The guests have come to the studio to disclose secret codes.
3. Caesar’s secret code was used for a hundred years after his death.
4. His code was a complicated one.
5. People were not able to break the code as they could not write.
6. The Arab mathematician al-Kindi lived in the 19th century.
7. His book on deciphering codes was written only in 1987.
8. He analysed how often codes were used in different languages.
9. The most frequent letter in English is 'I'.
10. Arthur Conan Doyle used numbers instead of letters.