Apex domains cannot have CNAME records.
Question and response
Question
dig mx.cloudflare.net. NS
Answer
mx.cloudflare.net. 5 NS arvind.ns.cloudflare.com.
mx.cloudflare.net. 5 NS tara.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0