Apex domains cannot have CNAME records.
Question and response
Question
dig b.root-servers.org. NS
Answer
b.root-servers.org. 5 NS ns-w.b.isi.edu.
b.root-servers.org. 5 NS ns-e.b.isi.edu.
b.root-servers.org. 5 NS ns-lax.b.root-servers.org.
Authority
The response has no authority records.
Additional
ns-lax.b.root-servers.org. 5 A 206.117.5.92
ns-lax.b.root-servers.org. 5 AAAA 2001:500:204:0:0:1:0:92
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0