Apex domains cannot have CNAME records.
Question and response
Question
dig cuhk.edu.hk. NS
Answer
cuhk.edu.hk. 5 NS dns2.cuhk.edu.hk.
cuhk.edu.hk. 5 NS dns3.cuhk.edu.hk.
cuhk.edu.hk. 5 NS ns4.gdnsec.com.
cuhk.edu.hk. 5 NS dns1.cuhk.edu.hk.
cuhk.edu.hk. 5 NS anyns.cuhk.edu.hk.
cuhk.edu.hk. 5 NS ns4.gdnsdef.com.
Authority
The response has no authority records.
Additional
dns1.cuhk.edu.hk. 5 A 137.189.10.13
dns2.cuhk.edu.hk. 5 A 137.189.6.22
anyns.cuhk.edu.hk. 5 A 204.61.216.69
dns1.cuhk.edu.hk. 5 AAAA 2405:3000:3:118:0:0:0:10d
dns2.cuhk.edu.hk. 5 AAAA 2405:3000:3:6:0:0:0:16
anyns.cuhk.edu.hk. 5 AAAA 2001:500:14:6069:ad:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0