Apex domains cannot have CNAME records.
Question and response
Question
dig rec.uba.ar. NS
Answer
rec.uba.ar. 5 NS ns2.uba.ar.
rec.uba.ar. 5 NS ns1.uba.ar.
Authority
The response has no authority records.
Additional
ns1.uba.ar. 4 A 157.92.1.1
ns2.uba.ar. 4 A 157.92.4.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0