You observe that a database performance has degraded over a period of time. While investigating the reason， you find that the size of the database buffer cache is not large enough to cache all the needed data blocks. Which advisory component would you refer to， in order to determine that required size of the database buffer cache？ （）

A. it can be used to concatenate two strings.
B. it can be used to find out the total length of the string.
C. it can be used for string manipulation and searching operations.
D. it can be used to format the output for a column or expression having string data.
E. it can be used to find and replace operations for a column or expression having string data.

A.to find the tax percentage for each of the employees
B.to list the name, job_id, and manager name for all the employees
C.to find the name, salary, and the department name of employees who are not working with Smith
D.to find the number of employees working for the Administrative department and earning less than 4000
E.to display name, salary, manager ID, and department name of all the employees, even if the employees do not have a department ID assigned