Explanation
http://docs.oracle.com/cd/E11882_01/backup.112/e10642/rcmconfb.htm#BRADV89399

Explanation
The alert log is a chronological log of messages and errors, and includes the following items:
*All internal errors (ORA-600), block corruption errors (ORA-1578), and deadlock errors (ORA-60) that occur
* Administrative operations, such as CREATE, ALTER, and DROP statements and STARTUP, SHUTDOWN, and ARCHIVELOG statements
* Messages and errors relating to the functions of shared server and dispatcher processes
* Errors occurring during the automatic refresh of a materialized view
* The values of all initialization parameters that had nondefault values at the time the database and instance start Note:
* The alert log file (also referred to as the ALERT.LOG) is a chronological log of messages and errors written out by an Oracle Database. Typical messages found in this file is: database startup, shutdown, log switches, space errors, etc. This file should constantly be monitored to detect unexpected messages and corruptions.

Explanation/Reference:
Explanation:
AB: You can make individual table columns invisible. Any generic access of a table does not show the invisible columns in the table. For example, the following operations do not display invisible columns in the output:
* SELECT * FROM statements in SQL
* DESCRIBE commands in SQL*Plus
* %ROWTYPE attribute declarations in PL/SQL
* Describes in Oracle Call Interface (OCI)
Incorrect:
Not D: You can make invisible columns visible.
You can make a column invisible during table creation or when you add a column to a table, and you can later alter the table to make the same column visible.
References:

Explanation
A: ASM_POWER_LIMIT specifies the maximum power on an Automatic Storage Management instance for disk rebalancing. The higher the limit, the faster rebalancing will complete. Lower values will take longer, but consume fewer processing and I/O resources.
D:
* Normally a separate process is fired up to do that rebalance. This will take a certain amount of time. If you want it to happen faster, fire up more processes. You tell ASM it can add more processes by increasing the rebalance power.
* ASMB
ASM Background Process
Communicates with the ASM instance, managing storage and providing statistics Incorrect:
Not B: A higher, not a lower, value of DISK_REPAIR_TIME would be helpful here.
Not E: If you implement database writer I/O slaves by setting the DBWR_IO_SLAVES parameter, you configure a single (master) DBWR process that has slave processes that are subservient to it. In addition, I/O slaves can be used to "simulate" asynchronous I/O on platforms that do not support asynchronous I/O or implement it inefficiently. Database I/O slaves provide non-blocking, asynchronous requests to simulate asynchronous I/O.

* The CREATE DATABASE ... ENABLE PLUGGABLE DATABASE SQL statement creates a new CDB. If you do not specify
the ENABLE PLUGGABLE DATABASE clause, then the newly created database is a non-CDB and can never contain
PDBs.
Along with the root (CDB$ROOT), Oracle Database automatically creates a seed PDB (PDB$SEED). The following
graphic shows a newly created CDB:

* Creating a PDB
Rather than constructing the data dictionary tables that define an empty PDB from scratch, and then populating its
Obj$ and Dependency$ tables, the empty PDB is created when the CDB is created. (Here, we use empty to mean
containing no customer-created artifacts.) It is referred to as the seed PDB and has the name PDB$Seed. Every CDB
non-negotiably contains a seed PDB; it is non-negotiably always open in read-only mode. This has no conceptual
significance; rather, it is just an optimization device. The create PDB operation is implemented as a special case of the
clone PDB operation.