Explanation/Reference:
Explanation:
Asynchronous Global Index Maintenance for DROP and TRUNCATE PARTITION
This feature enables global index maintenance to be delayed and decoupled from a DROP and TRUNCATE partition without making a global index unusable. Enhancements include faster DROP and TRUNCATE partition operations and the ability to delay index maintenance to off-peak time.

* PGA memory allocation depends on whether the database uses dedicated or shared server connections.

Note:
*System global area (SGA) The SGA is a group of shared memory structures, known as SGA components, that contain data and control information for one Oracle Database instance. The SGA is shared by all server and background processes. Examples of data stored in the SGA include cached data blocks and shared SQL areas.
*Program global area (PGA)
A PGA is a memory region that contains data and control information for a server process. It is nonshared memory created by Oracle Database when a server process is started. Access to the PGA is exclusive to the server process. There is one PGA for each server process. Background processes also allocate their own PGAs. The total memory used by all individual PGAs is known as the total instance PGA memory, and the collection of individual PGAs is referred to as the total instance PGA, or just instance PGA. You use database initialization parameters to set the size of the instance PGA, not individual PGAs.
Reference: Oracle Database Concepts 12c

Explanation
To perform operations on a single PDB, you can connect as target either to the root or directly to the PDB.
* (A) If you connect to the root, you must use the PLUGGABLE DATABASE syntax in your RMAN commands. For example, to back up a PDB, you use the BACKUP PLUGGABLE DATABASE command.
* (C)If instead you connect directly to a PDB, you can use the same commands that you would use when connecting to a non-CDB. For example, to back up a PDB, you would use the BACKUP DATABASE command.

Explanation
About Startup Dependencies
Oracle Restart ensures that Oracle components are started in the proper order, in accordance with component dependencies. For example, if database files are stored in Oracle ASM disk groups, then before starting the database instance, Oracle Restart ensures that the Oracle ASM instance is started and the required disk groups are mounted. Likewise, if a component must be shut down, Oracle Restart ensures that dependent components are cleanly shut down first.
Oracle Restart also manages the weak dependency between database instances and the Oracle Net listener (the listener): When a database instance is started, Oracle Restart attempts to start the listener. If the listener startup fails, then the database is still started. If the listener later fails, Oracle Restart does not shut down and restart any database instances.
References:
http://docs.oracle.com/cd/E16655_01/server.121/e17636/restart.htm#ADMIN12710

Asynchronous Global Index Maintenance for DROP and TRUNCATE PARTITION
This feature enables global index maintenance to be delayed and decoupled from a DROP and TRUNCATE partition without making a global index unusable. Enhancements include faster DROP and TRUNCATE partition operations and the ability to delay index maintenance to off-peak time.