Which of the following is not a problem when rollback is needed as a means of recovery and concurrent update processes have altered the damaged database?

a. All processes that update the corrupted data items must be identified so they can be locked out when an error is discovered

b. It might be difficult to determine the consequences of error so the effects of the damage can be undone

c. Rollback cannot be accomplished if afterimages have been damaged

d. Rollback may be pointless if too many other processes have accessed the data in error before the error has been discovered