Apex domains cannot have CNAME records.
Question and response
Question
dig virtua.com.br. NS
Answer
virtua.com.br. 5 NS dns2.virtua.com.br.
virtua.com.br. 5 NS dns3.virtua.com.br.
virtua.com.br. 5 NS dns1.virtua.com.br.
Authority
The response has no authority records.
Additional
dns1.virtua.com.br. 5 A 201.6.4.15
dns2.virtua.com.br. 5 A 201.6.4.61
dns3.virtua.com.br. 5 A 189.6.48.3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0