Apex domains cannot have CNAME records.
Question and response
Question
dig btc-net.bg. NS
Answer
btc-net.bg. 5 NS pns.btc-net.bg.
btc-net.bg. 5 NS sns.btc-net.bg.
Authority
The response has no authority records.
Additional
pns.btc-net.bg. 5 A 212.39.90.73
sns.btc-net.bg. 5 A 212.39.90.77
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0