053-628

The database is running in the ARCHIVELOG mode. It has three redo log groups with one member
each. One of the redo log groups has become corrupted. You have issued the following command during the recovery of a damaged redo log file:
ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP 3;
Which action should you perform immediately after using this command?
A. You should perform a log switch
B. You should make a backup of the database
C. You should switch the database to the NONARCHIVELOG mode
D. You should shut down the database instance and perform a complete database recovery

使用 UNARCHIVED 选项来清理日志组的话，不会对其进行归档，直接清理然后日志组可用。这样如果其没归档的话，那么那些需要归档的备份就不能恢复了（出现了断点）。在 alter 日志中也会收到一条告警信息，表明不能进行恢复。所以 B 正确，你应该立即执行一个备份，因为之前的备份已经失效。
clear 语句克服了 2 种不能删除日志组的情况：
1.只有 2 个日志组
2.要删除的日志组是当前日志组
如果要清理的日志组并未归档，那么需要使用 UNARCHIVED 选项来强制清理。
C 明显错误，D 也错误，日志文件损坏时不用进行数据库恢复的。 A 选项可以执行但是和题意不符，错。