$[2\%]$ IBM's indexed sequential access method $($ISAM$)$

$($a$),$ uses an index file for sequential access

$($b$).$ uses a small master index $($kept in memory$)$ that points to disk blocks of a secondary index, while the secondary

index blocks point to the actual file blocks

$($c$).$ use the pointers $($kept in memory$)$ of the actual file blocks

$($d$).$ uses the pointers $($kept in memory$)$ that points to disk blocks of a master index which points to disk blocks of

a secondary index, while the secondary index blocks point to the actual file blocks