Apex domains cannot have CNAME records.
Question and response
Question
dig dc.ctc.ad.jp. NS
Answer
dc.ctc.ad.jp. 5 NS ns018-a4j8cjalqsdc7e9m.a.d-53.net.
dc.ctc.ad.jp. 5 NS ns018-a4j8cjalqsdc7e9m.a.d-53.info.
dc.ctc.ad.jp. 5 NS ns3.ctc.jp.
dc.ctc.ad.jp. 5 NS ns018-a4j8cjalqsdc7e9m.a.d-53.jp.
Authority
The response has no authority records.
Additional
ns3.ctc.jp. 5 A 13.230.125.104
ns3.ctc.jp. 5 AAAA 2406:da14:463:7801:dbb3:ecb4:d989:e9bc
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0