Which two elements comprise the backbone area while designing a new network utilizing IS-IS as the interior gateway protocol (choose two)

Which two elements comprise the backbone area while designing a new network utilizing IS-IS as the interior gateway protocol (choose two)
A . contiguous chain of Level 2 capable routers
B . contiguous chain of Level1/Level 2 capable routers
C . set of Level 2 configured interfaces
D . contiguous Level 1 capable routers
E . set of Level 1 configured interfaces

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Answer: A, B

Explanation:

https://www.cisco.com/en/US/products/ps6599/products_white_paper09186a00800a3e6f.shtml

"Backbone IS-IS does not have a backbone area like the OSPF area 0. The IS-IS backbone is a contiguous collection of Level 2-capable routers, each of which can be in a different area"

http://www.ciscopress.com/articles/article.asp?p=26850&seqNum=3

"In most designs with routing hierarchy, the Level 2 routers are also Level 1 routers by virtue of their

identification with a certain area. Therefore, in IS-IS, a router can function as Level 1-only or Level 2-only and possibly as both Level 1 and Level 2 (Level 1-2). Level 1-2 routers act as border routers to their respective areas, providing connectivity to other areas. The Level 2 backbone is essentially a virtual IS-IS area consisting of routers engaged in Level 2 routing"