Under 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