Apex domains cannot have CNAME records.
Question and response
Question
dig ac.lk. NS
Answer
ac.lk. 5 NS ns2.ac.lk.
ac.lk. 5 NS lk.communitydns.net.
ac.lk. 5 NS ns1.ac.lk.
ac.lk. 5 NS ns3.ac.lk.
Authority
The response has no authority records.
Additional
ns1.ac.lk. 5 A 192.248.1.162
ns2.ac.lk. 5 A 192.248.1.161
ns3.ac.lk. 5 A 192.248.121.3
ns1.ac.lk. 5 AAAA 2401:dd00:1:0:0:0:0:162
ns2.ac.lk. 5 AAAA 2401:dd00:1:0:0:0:0:161
ns3.ac.lk. 5 AAAA 2401:dd00:1:1:0:0:0:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0