Apex domains cannot have CNAME records.
Question and response
Question
dig b.isi.edu. NS
Answer
b.isi.edu. 5 NS ns-w.b.isi.edu.
b.isi.edu. 5 NS ns-lax.b.root-servers.org.
b.isi.edu. 5 NS ns-e.b.isi.edu.
Authority
The response has no authority records.
Additional
ns-e.b.isi.edu. 5 A 128.9.63.136
ns-w.b.isi.edu. 5 A 128.9.28.251
ns-lax.b.root-servers.org. 5 A 206.117.5.92
ns-e.b.isi.edu. 5 AAAA 2001:470:7:b8:1:0:8009:3f88
ns-w.b.isi.edu. 5 AAAA 2001:1878:401:0:0:0:8009:1cfb
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