Apex domains cannot have CNAME records.
Question and response
Question
dig BG.AC.RS. NS
Answer
BG.AC.RS. 5 NS gaea.rcub.BG.AC.RS.
BG.AC.RS. 5 NS ns.rcub.BG.AC.RS.
BG.AC.RS. 5 NS ban.junis.ni.AC.RS.
BG.AC.RS. 5 NS ns.etf.BG.AC.RS.
BG.AC.RS. 5 NS ns.unic.kg.AC.RS.
BG.AC.RS. 5 NS ns1.uns.AC.RS.
Authority
The response has no authority records.
Additional
ns.etf.bg.ac.rs. 5 A 147.91.8.6
ns.rcub.bg.ac.rs. 5 A 147.91.1.5
ns.unic.kg.ac.rs. 5 A 147.91.209.2
ban.junis.ni.ac.rs. 5 A 160.99.1.1
ns1.uns.ac.rs. 5 A 147.91.173.4
gaea.rcub.bg.ac.rs. 5 A 147.91.1.7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0