Apex domains cannot have CNAME records.
Question and response
Question
dig gruporscom.com. NS
Answer
gruporscom.com. 5 NS dns2.artinfo.com.br.
gruporscom.com. 5 NS dns1.artinfo.com.br.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0