Apex domains cannot have CNAME records.
Question and response
Question
dig bg.ac.RS. NS
Answer
bg.ac.rs. 1 NS ns.unic.kg.ac.rs.
bg.ac.rs. 1 NS ns1.uns.ac.rs.
bg.ac.rs. 1 NS ns.etf.bg.ac.rs.
bg.ac.rs. 1 NS gaea.rcub.bg.ac.rs.
bg.ac.rs. 1 NS ns.rcub.bg.ac.rs.
bg.ac.rs. 1 NS ban.junis.ni.ac.rs.
Authority
The response has no authority records.
Additional
ns.etf.bg.ac.rs. 1 A 147.91.8.6
ns.rcub.bg.ac.rs. 1 A 147.91.1.5
ns.unic.kg.ac.rs. 1 A 147.91.209.2
ban.junis.ni.ac.rs. 1 A 160.99.1.1
ns1.uns.ac.rs. 1 A 147.91.173.4
gaea.rcub.bg.ac.rs. 1 A 147.91.1.7
ns.rcub.bg.ac.rs. 1 AAAA 2001:4170:0:1:0:0:0:5
ns.unic.kg.ac.rs. 1 AAAA 2001:4170:3000:1:0:0:0:2
ban.junis.ni.ac.rs. 1 AAAA 2001:4170:2801:0:0:0:0:1
ns1.uns.ac.rs. 1 AAAA 2001:4170:2000:1:0:0:0:4
gaea.rcub.bg.ac.rs. 1 AAAA 2001:4170:0:1:0:0:0:7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0