Which prerequisite must be met before accomplishing the backup?

Examine the following RMAN command:

RMAN> CONFIGURE ENCRYPTION FOR DATABASE ON;
RMAN> BACKUP DATABASE PLUS ARCHIVELOG;

Which prerequisite must be met before accomplishing the backup?

A.
Provide a password for the encryption

B.
Set up an Oracle wallet for the encryption

C.
No setup is required as it is a default encryption method

D.
Both Oracle wallet and password must be set up for the encryption

Explanation:
http://download.oracle.com/docs/cd/B28359_01/backup.111/b28270/rcmconfa.htm

To configure the environment so that all RMAN backups are encrypted:
Set up the Oracle wallet as explained in Oracle Database Advanced Security Administrator’s Guide.

Issue the following RMAN command:

CONFIGURE ENCRYPTION FOR DATABASE ON;

At this stage, all RMAN backup sets created by this database will use transparent encryption by default.