Apex domains cannot have CNAME records.
Question and response
Question
dig ksc.co.th. NS
Answer
ksc.co.th. 5 NS ns2.ksc.co.th.
ksc.co.th. 5 NS ns.ksc.co.th.
Authority
The response has no authority records.
Additional
ns.ksc.co.th. 5 A 203.155.33.1
ns2.ksc.co.th. 5 A 202.44.144.33
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0