Apex domains cannot have CNAME records.
Question and response
Question
dig 21vianet.com. NS
Answer
21vianet.com. 5 NS vip4.alidns.com.
21vianet.com. 5 NS vip3.alidns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0