Apex domains cannot have CNAME records.
Question and response
Question
dig sk. NS
Answer
sk. 5 NS g.tld.sk.
sk. 5 NS e.tld.sk.
sk. 5 NS f.tld.sk.
sk. 5 NS h.tld.sk.
Authority
The response has no authority records.
Additional
e.tld.sk. 129517 A 194.169.218.16
f.tld.sk. 129517 A 185.24.64.16
g.tld.sk. 129517 A 212.18.248.16
h.tld.sk. 129517 A 212.18.249.16
e.tld.sk. 129517 AAAA 2001:67c:13cc:0:0:0:1:16
f.tld.sk. 129517 AAAA 2a04:2b00:13cc:0:0:0:1:16
g.tld.sk. 129517 AAAA 2a04:2b00:13ee:0:0:0:0:16
h.tld.sk. 129517 AAAA 2a04:2b00:13ff:0:0:0:0:16
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0