What could be the source of the problem when you encounter a situation where a 7910 IP phone registers is Cisco CME, but extension numbers do not appear?

A.
ephone-dn has not been mapped to ephone.

B.
An undefined phone type for 7910 under ephone.

C.
There is an authentication failure during auto-registration.

D.
Cisco CME and 7910 need to be in same device pool.