Explanation : we must undergo log record 6 to set B to 10000 and then redo log records 2 and 3 because system faols before commit operation.so we need to undone active transactions(T2) and redo committed transactions(T1)
note: here we are not using checkpoints.
when a system with concurrent transactions crashes and recovers,it behaves in the following manner–
–> The recovery system reads the logs backwards from the end to last checkpoint.
–> It maintains two lists, an undo -list ,redo-list.
–> If the recovery system sees a log with but no commit or abort log found,it puts the transaction in undo-list.
–>All the transactions in the undo list are then undone and their logs are removed.All the transactions in the redo-list and their previous logs are removed and then redone before saving their logs.
so the answer is B redo log records 2 and 3 and undo log record 6.