Apex domains cannot have CNAME records.
Question and response
Question
dig dns.usp.br. NS
Answer
dns.usp.br. 5 NS b.dns.usp.br.
dns.usp.br. 5 NS a.dns.usp.br.
dns.usp.br. 5 NS c.dns.usp.br.
Authority
The response has no authority records.
Additional
a.dns.usp.br. 4 A 200.144.248.208
b.dns.usp.br. 4 A 200.144.249.208
c.dns.usp.br. 4 A 177.90.0.18
a.dns.usp.br. 4 AAAA 2001:12d0:c000:0:0:0:0:208
b.dns.usp.br. 4 AAAA 2001:12d0:c080:0:0:0:0:208
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0