Apex domains cannot have CNAME records.
Question and response
Question
dig btcltelesheba.gov.bd. NS
Answer
btcltelesheba.gov.bd. 5 NS slave.btcl.net.bd.
btcltelesheba.gov.bd. 5 NS dns.btcl.net.bd.
Authority
The response has no authority records.
Additional
dns.btcl.net.bd. 5 A 123.49.12.148
slave.btcl.net.bd. 5 A 123.49.12.149
dns.btcl.net.bd. 5 AAAA 2407:5000:b:0:0:0:0:2
slave.btcl.net.bd. 5 AAAA 2407:5000:b:0:0:0:0:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0