How do BGP confederations reduce the requirements for a full-mesh iBGP configuration?

A.
By splitting the autonomous system into a number of smaller autonomous systems and using iBGP sessions among them.

B.
By modifying the iBGP split horizon rule.

C.
By modifying the eBGP split horizon rule.

D.
The combining multiple IBGP sessions into a single IBGP session over loopback interface.

E.
By synchronizing the IBGP updates to the IGP updates.