Apex domains cannot have CNAME records.
Question and response
Question
dig 210.in-addr.arpa. NS
Answer
210.in-addr.arpa. 5 NS rirns.arin.net.
210.in-addr.arpa. 5 NS apnic.authdns.ripe.net.
210.in-addr.arpa. 5 NS ns2.apnic.net.
210.in-addr.arpa. 5 NS ns3.lacnic.net.
210.in-addr.arpa. 5 NS ns1.apnic.net.
Authority
The response has no authority records.
Additional
ns1.apnic.net. 3 A 203.119.42.53
ns2.apnic.net. 3 A 203.119.95.53
ns3.lacnic.net. 3 A 200.3.13.14
apnic.authdns.ripe.net. 3 A 193.0.9.9
rirns.arin.net. 3 A 199.253.249.53
ns1.apnic.net. 3 AAAA 2001:dd8:e:0:0:0:0:53
ns2.apnic.net. 3 AAAA 2001:ddd:0:0:0:0:0:53
ns3.lacnic.net. 3 AAAA 2001:13c7:7002:3000:0:0:0:14
apnic.authdns.ripe.net. 3 AAAA 2a13:27c0:30:0:0:0:0:9
rirns.arin.net. 3 AAAA 2620:38:2000:0:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0