Explanation/Reference:
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.

The benefits of Oracle Multitenant are brought by implementing a pure deployment choice. The following list calls out the most compelling examples.
* High consolidation density. (E)
The many pluggable databases in a single multitenant container database share its memory and background processes, letting you operate many more pluggable databases on a particular platform than you can single databases that use the old architecture. This is the same benefit that schema-based consolidation brings.
* Rapid provisioning and cloning using SQL.
* New paradigms for rapid patching and upgrades. (D, not B)
The investment of time and effort to patch one multitenant container database results in patching all of its many pluggable databases. To patch a single pluggable database, you simply unplug/plug to a multitenant container database at a different Oracle Database software version.
* (C, not A) Manage many databases as one.
By consolidating existing databases as pluggable databases, administrators can manage many databases as one. For example, tasks like backup and disaster recovery are performed at the multitenant container database level.
* Dynamic between pluggable database resource management. In Oracle Database 12c, Resource Manager is extended with specific functionality to control the competition for resources between the pluggable databases within a multitenant container database.
Note:
* Oracle Multitenant is a new option for Oracle Database 12c Enterprise Edition that helps customers reduce IT costs by simplifying consolidation, provisioning, upgrades, and more. It is supported by a new architecture that allows a multitenant container database to hold many pluggable databases. And it fully complements other options, including Oracle Real Application Clusters and Oracle Active Data Guard. An existing database can be simply adopted, with no change, as a pluggable database; and no changes are needed in the other tiers of the application.

With adaptive SQL plan management, DBAs no longer have to manually run the verification or evolve process for non-accepted plans. When automatic SQL tuning is in COMPREHENSIVE mode, it runs a verification or evolve process for all SQL statements that have non-accepted plans during the nightly maintenance window. If the non-accepted plan performs better than the existing accepted plan (or plans) in the SQL plan baseline, then the plan is automatically accepted and becomes usable by the optimizer. After the verification is complete, a persistent report is generated detailing how the non-accepted plan performs compared to the accepted plan performance. Because the evolve process is now an AUTOTASK, DBAs can also schedule their own evolve job at end time.
Note:
*The optimizer is able to adapt plans on the fly by predetermining multiple subplans for portions of the plan.
*Adaptive plans, introduced in Oracle Database 12c, enable the optimizer to defer the final plan decision for a statement until execution time. The optimizer instruments its chosen plan (the default plan) with statistics collectors so that it can detect at runtime, if its cardinality estimates differ greatly from the actual number of rows seen by the operations in the plan. If there is a significant difference, then the plan or a portion of it will be automatically adapted to avoid
suboptimal performance on the first execution of a SQL statement.
Reference: SQL Plan Management with Oracle Database 12c

The statistics collected and processed by AWR include:
Time model statistics based on time usage for activities, displayed in the V$SYS_TIME_MODEL and V$SESS_TIME_MODEL views
http://docs.oracle.com/cd/B28359_01/server.111/b28274/autostat.htm
http://docs.oracle.com/cd/E11882_01/server.112/e41573/autostat.htm#PFGRF02601