Apex domains cannot have CNAME records.
Question and response
Question
dig top. NS
Answer
top. 5 NS c.zdnscloud.com.
top. 5 NS j.zdnscloud.com.
top. 5 NS i.zdnscloud.cn.
top. 5 NS f.zdnscloud.cn.
top. 5 NS a.zdnscloud.cn.
top. 5 NS d.zdnscloud.com.
top. 5 NS g.zdnscloud.com.
top. 5 NS b.zdnscloud.cn.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0