What is a significant difference between host and zone keys generated by dnssec-keygen?

A.
There is no difference.

B.
Zone key files contain a public and private key.

C.
Host keys files contain a public and private key.

D.
Host keys must always be generated if DNSSEC is used; zone keys are optional

E.
Zone keys must always be generated if DNSSEC is used; host keys are optional

Explanation: