Apex domains cannot have CNAME records.
Question and response
Question
dig dk. NS
Answer
dk. 5 NS b.nic.dk.
dk. 5 NS s.nic.dk.
dk. 5 NS c.nic.dk.
dk. 5 NS l.nic.dk.
dk. 5 NS h.nic.dk.
dk. 5 NS t.nic.dk.
Authority
The response has no authority records.
Additional
b.nic.dk. 163129 A 193.163.102.222
c.nic.dk. 162334 A 185.159.199.45
h.nic.dk. 172235 A 46.30.126.214
l.nic.dk. 163129 A 130.226.213.138
t.nic.dk. 172235 A 185.43.134.16
b.nic.dk. 163129 AAAA 2a01:630:0:80:0:0:0:53
c.nic.dk. 162334 AAAA 2620:10a:80ac:0:0:0:0:45
h.nic.dk. 172235 AAAA 2a00:1b18:1:2:0:0:0:53
l.nic.dk. 163129 AAAA 2001:878:0:e000:82:e2:d5:8a
t.nic.dk. 172235 AAAA 2001:148f:fffd:0:0:0:0:16
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0