16. Enforcing serializability in concurrent schedules ensures which two of the four desired properties for transactions? (a) Atomicity and consistency (b) Atomicity and isolation (c) Atomicity and durability (d) Consistency and isolation (e) Consistency and durability (f) Isolation and durability 17. The recovery manager ensures which two of the four desired properties for transactions? (a) Atomicity and consistency (b) Atomicity and isolation (c) Atomicity and durability (d) Consistency and isolation (e) Consistency and durability (f) Isolation and durability 18. Suppose a database is read-only-no transactions change any data in the database. If serializability must be supported, which of the following is true? (a) No locking is necessary. (b) Only read locks are necessary and they need to be held until end of transaction. (c) Only read locks are necessary, but they can be released as soon as the read is complete. (d) Both read and write locks are necessary and locking must be done in two phases. (e) None of the above. 19. Checkpointing is a technique that can reduce recovery time after a crash. Which of the following is true? (a) When recovering, the log only needs to be scanned back to the most recent checkpoint. (b) Checkpointing is automatically performed after every transaction commit. (c) Checkpoints are saved after every update to the database. (d) a and c (e) a, b, and c 20. Which of the following is true about updated pages in the buffer pool? (a) Updated pages must be written immediately after the update. (b) Updated pages must be written after a transaction commits but before the transaction log is written to disk. (c) Updated pages must be written after a transaction commits but after the transaction log is written to disk. (d) An updated page must be written when it is swapped out of the buffer pool