An existing server contains a single RAID 5 array with six drives, all 9 GB in size. One drive fails. It is not possible to obtain a 9 GB drive to replace it. The manufacturer supplies an 18 GB drive, claiming it is compatible with this server. What is the usable size of the array after installing the 18 GB drive?

A.
36 GB

B.
45 GB

C.
54 GB

D.
27 GB

E.
63 GB