A new administrator makes changes to the zone file on the primary DNS server and now the secondary DNS server fails to synchronize. The secondary server is rebooted but this fails to help. Which failure caused this problem?

A.
failure to update the NS record of the updated zone file

B.
failure to update the time record of the updated zone file

C.
failure to update the serial number of the updated zone file

D.
failure to update the SOA ttl number of the updated zone file