Which statement about license keys for PIX Firewalls is true?

Which statement about license keys for PIX Firewalls is true?

A.
License keys are specific to the PIX Firewall software versions.

B.
License keys are not required for any of the PIX Firewall software versions.

C.
License keys exist for the PIX Firewall 515E software version only.

D.
License keys are not specific to a particular PIX Firewall software version.