Cannot enable FileVault 2

A customer installed OS X Lion on his iMac, but cannot enable FileVault 2. What is a likely cause of this issue?
A. The iMac does not have sufficient RAM.
B. The hard drive does not have the correct partition scheme.
C. FileVault 2 was not chosen as a custom option during the OS X Lion install.
D. The customer continued the install of OS X Lion, despite a warning that the Recovery HD could not be created.
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