What are IKE Phase 1 Exchange (Main Mode) messages 3 and 4 used for?

What are IKE Phase 1 Exchange (Main Mode) messages 3 and 4 used for?

A.
generate SKEYID_e, which is used to encrypt IKE messages

B.
generate SKEYID_a, which is used to provide data integrity and authentication to IKE messages

C.
exchange authentication information (pre-shared key)

D.
exchange information that is required for key generation using Diffie-Hellman (DH)

E.
authenticate the digital signature (certifications)