Apex domains cannot have CNAME records.
Question and response
Question
dig algartelecom.com.br. NS
Answer
algartelecom.com.br. 5 NS nspar.ctbc.com.br.
algartelecom.com.br. 5 NS nssar.ctbc.com.br.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0