Apex domains cannot have CNAME records.
Question and response
Question
dig Level3.com. NS
Answer
Level3.com. 5 NS ns2.Level3.net.
Level3.com. 5 NS ns1.Level3.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0