Which statements about the MEMORY_TARGET initialization parameter are true？（） 

• A、 MEMORY_TARGET can be increased up to the value of MEMORY_MAX_TARGET， if  MEMORY_MAX_TARGET is set to a value greater than zero
• B、 MEMORY_MAX_TARGET defaults to a value of zero if MEMORY_TARGET is not set
• C、 MEMORY_TARGET represents the total amount of memory that can be allocated to SGA and PGA memory structures.
• D、 MEMORY_TARGET is static and cannot be modified without shutting down the instance