ANSWER:
A, D: Tape drives or a network hard drive could be used for backups, and would keep the data safe in the event of a hard disk failure. Since the computer has a single hard disk, storing it to the local hard drive wouldn't keep the data that's been backed up safe. A CD-ROM is a "WORM" device-Write Once, Read Many-and is capable of only reading data. A writable CD-ROM would be required to write the data, and would make a poor method of storage as each backup would require a new disk.
Answer in Depth...