What is the usable disk space of a RAID 5 array of five 18GB drives with one drive dedicated as a spare?

What is the usable disk space of a RAID 5 array of five 18GB drives with one drive dedicated as a
spare?

A.
18GB

B.
36GB

C.
54GB

D.
72GB

E.
90GB

Explanation:
See http://en.wikipedia.org/wiki/RAID
RAID5 capacity is available disks – 1. 5 disks – 1 – 1 spare = 3 disks. 3 x 18 = 54