Explanation:
The database can be open without any restrictions during an open database backup.
Incorrect Answers:
1: The database is fully accessible to all users.
2: The database must be operating in ARCHIVELOG mode when performing an open database backup.
4: The database table spaces should be in BEGIN BACKUP mode.
Oracle 8, DBA Certification Exam Guide, Jason S. Couchman, p. 587-590
Chapter 12: Overview of Backup and Recovery

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