Which method could be used to improve the scalability of IS-IS used as the IBGP of an ISP?

Which method could be used to improve the scalability of IS-IS used as the IBGP of an ISP?

A.
Use stub or totally stubby nonbackbone IS-IS areas.

B.
Use IS-IS route leaking into the nonbackbone IS-IS areas.

C.
Use IS-IS CSNP on Level-1-2 circuits.

D.
Use IS-IS pseudo-node on Level-1-2 circuits.

E.
Use proper interarea route summarization.

F.
Use proper external route summarization when redistributing BGP routes into IS-IS.