Which of the following is suitable as a description of the problems and solutions for improving the performance of a system that uses relational databases?
a) When an SQL statement was executed, a full search occurred along the access path to which an index could not be used. Therefore, the database was re-organized.
b) I/O waiting time increased due to concentrated accesses, so the database was partitioned into multiple disks.
c) It was found that accessing was concentrated only to certain specific rows. Therefore, the index was re-set.
d) The amount of data placed in an overflow area increased due to updates and additions during operation over an extended time period. Therefore, the block size was reduced.