what conditions does a secondary name server request a zone transfer from a primary name server?

Under what conditions does a secondary name server request a zone transfer from a primary name server?

A.
When a primary SOA is higher that a secondary SOA

B.
When a secondary SOA is higher that a primary SOA

C.
When a primary name server has had its service restarted

D.
When a secondary name server has had its service restarted

E.
When the TTL falls to zero

Explanation:
Understanding DNS is critical to meeting the requirements of the CEH. When the serial number that is within the SOA record of the primary server is higher than the Serial number within the SOA record of the secondary DNS server, a zone transfer will take place.