Apex domains cannot have CNAME records.
Question and response
Question
dig ust.hk. NS
Answer
ust.hk. 5 NS a3-66.akam.net.
ust.hk. 5 NS a9-66.akam.net.
ust.hk. 5 NS a8-67.akam.net.
ust.hk. 5 NS a1-251.akam.net.
ust.hk. 5 NS a20-65.akam.net.
ust.hk. 5 NS a28-64.akam.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0