Apex domains cannot have CNAME records.
Question and response
Question
dig 5x00.com. NS
Answer
5x00.com. 5 NS reutov.ens.mail.ru.
5x00.com. 5 NS angarsk.ens.mail.ru.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0