Apex domains cannot have CNAME records.
Question and response
Question
dig ac. NS
Answer
ac. 5 NS c0.nic.ac.
ac. 5 NS a2.nic.ac.
ac. 5 NS a0.nic.ac.
ac. 5 NS b0.nic.ac.
Authority
The response has no authority records.
Additional
a0.nic.ac. 154372 A 65.22.160.1
a2.nic.ac. 154372 A 65.22.163.1
b0.nic.ac. 154372 A 65.22.161.1
c0.nic.ac. 154372 A 65.22.162.1
a0.nic.ac. 154372 AAAA 2a01:8840:9e:0:0:0:0:1
a2.nic.ac. 154372 AAAA 2a01:8840:a1:0:0:0:0:1
b0.nic.ac. 154372 AAAA 2a01:8840:9f:0:0:0:0:1
c0.nic.ac. 154372 AAAA 2a01:8840:a0:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0