When would a certificate authority revoke a certificate?

A.
If the user�s public key has become compromised

B.
If the user changed over to using the PEM model that uses a web of trust

C.
If the user�s private key has become compromised

D.
If the user moved to a new location

Explanation:
The reason that a certificate is revoked is to warn others who use
that persons public key that they should no longer trust the public key. For some
reason that public key is no longer bound to that particular individuals identity.
This could be because an employee left the company, a person changed their name and
needed a new certificate, but most likely it is because the persons private key was
compromised.