Keyword: shows the difference.
*Full ADDM analysis across two AWR snapshot periods Detects causes, measure effects, then correlates them Causes: workload changes, configuration changes Effects: regressed SQL, reach resource limits (CPU, I/O, memory, interconnect) Makes actionable recommendations along with quantified impact
*Identify what changed / Configuration changes, workload changes
*Performance degradation of the database occurs when your database was performing optimally
in the past, such as 6 months ago, but has gradually degraded to a point where it becomes noticeable to the users. The Automatic Workload Repository (AWR) Compare Periods report enables you to compare database performance between two periods of time.
While an AWR report shows AWR data between two snapshots (or two points in time), the AWR Compare Periods report shows the difference (ABE) between two periods (or two AWR reports with a total of four snapshots). Using the AWR Compare Periods report helps you to identify detailed performance attributes and configuration settings that differ between two time periods.
Reference: Resolving Performance Degradation Over Time

Incorrect:
Not B: The online move data file operation may get aborted if the standby recovery process takes
the data file offline, shrinks the file (not B), or drops the tablespace.
Not D: The online move data file operation cannot be executed on physical standby while standby
recovery is running in a mounted but not open instance.
Note:
You can move the location of an online data file from one physical file to another physical file while
the database is actively accessing the file. To do so, you use the SQL statement ALTER
DATABASE MOVE DATAFILE.
An operation performed with the ALTER DATABASE MOVE DATAFILE statement increases the
availability of the database because it does not require that the database be shut down to move
the location of an online data file. In releases prior to Oracle Database 12c Release 1 (12.1), you
could only move the location of an online data file if the database was down or not open, or by first
taking the file offline.
You can perform an online move data file operation independently on the primary and on the
standby (either physical or logical). The standby is not affected when a data file is moved on the
primary, and vice versa.
Reference: Oracle Data Guard Concepts and Administration 12c, Moving the Location of Online
Data Files

Explanation/Reference:
Explanation:
http://docs.oracle.com/cd/E11882_01/server.112/e25494/dbrm.htm#ADMIN11844

Explanation
A, D: The value for the disk group COMPATIBLE.ASM attribute determines the minimum software version for an Oracle ASM instance that can use the disk group. This setting also affects the format of the data structures for the Oracle ASM metadata on the disk.
B: The value for the disk group COMPATIBLE.RDBMS attribute determines the minimum COMPATIBLE database initialization parameter setting for any database instance that is allowed to use the disk group. Before advancing the COMPATIBLE.RDBMS attribute, ensure that the values for the COMPATIBLE initialization parameter for all of the databases that access the disk group are set to at least the value of the new setting for COMPATIBLE.RDBMS.
For example, if the COMPATIBLE initialization parameters of the databases are set to either 11.1 or 11.2, then COMPATIBLE.RDBMS can be set to any value between 10.1 and 11.1 inclusively.