Apex domains cannot have CNAME records.
Question and response
Question
dig ctihk.com. NS
Answer
ctihk.com. 5 NS ns2o.ctihk.com.
ctihk.com. 5 NS ns1o.ctihk.com.
Authority
The response has no authority records.
Additional
ns1o.ctihk.com. 5 A 203.80.96.8
ns2o.ctihk.com. 5 A 203.186.94.21
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0