Apex domains cannot have CNAME records.
Question and response
Question
dig bd. NS
Answer
bd. 5 NS dns.bd.
bd. 5 NS surma.btcl.net.bd.
bd. 5 NS bd-ns.anycast.pch.net.
bd. 5 NS jamuna.btcl.net.bd.
Authority
The response has no authority records.
Additional
dns.bd. 156278 A 123.49.12.112
surma.btcl.net.bd. 172656 A 203.112.194.232
jamuna.btcl.net.bd. 145232 A 203.112.194.231
dns.bd. 156278 AAAA 2407:5000:88:5:0:0:0:3
surma.btcl.net.bd. 172656 AAAA 2407:5000:88:4:0:0:0:232
jamuna.btcl.net.bd. 145232 AAAA 2407:5000:88:4:0:0:0:231
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0