Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-cn-21.cn. NS
Answer
awsdns-cn-21.cn. 4 NS g-ns-intl-3934.awsdns-cn-21.cn.
awsdns-cn-21.cn. 4 NS g-ns-1292.awsdns-cn-21.cn.
awsdns-cn-21.cn. 4 NS g-ns-3934.awsdns-cn-21.cn.
awsdns-cn-21.cn. 4 NS g-ns-237.awsdns-cn-21.cn.
awsdns-cn-21.cn. 4 NS g-ns-2878.awsdns-cn-21.cn.
awsdns-cn-21.cn. 4 NS g-ns-intl-237.awsdns-cn-21.cn.
Authority
The response has no authority records.
Additional
g-ns-237.awsdns-cn-21.cn. 4 A 52.82.176.237
g-ns-1292.awsdns-cn-21.cn. 4 A 52.82.181.12
g-ns-2878.awsdns-cn-21.cn. 4 A 54.222.35.62
g-ns-3934.awsdns-cn-21.cn. 4 A 54.222.39.94
g-ns-intl-237.awsdns-cn-21.cn. 4 A 52.46.184.237
g-ns-intl-3934.awsdns-cn-21.cn. 4 A 52.46.183.94
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0