How to encrypt their existing Time Machine backup

A customer decides to encrypt their existing Time Machine backup. How should they proceed?
A. Erase the drive using the Disk Utility then start a new Time Machine backup.
B. Enable FileVault 2 on the startup disk to automatically encrypt the Time Machine backup.
C. Get info on the Time Machine backup drive then select the enable encryptioncheckbox.
D. Use the Time Machine preferences to re-select the backup drive then select theencrypt backup diskcheckbox.
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