Apex domains cannot have CNAME records.
Question and response
Question
dig telefonica.com. NS
Answer
telefonica.com. 3 NS dns1.movistar.es.
telefonica.com. 3 NS dns2.movistar.es.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0