Apex domains cannot have CNAME records.
Question and response
Question
dig 166.com. NS
Answer
166.com. 5 NS ns2.166.com.
166.com. 5 NS ns3.166.com.
166.com. 5 NS ns6.166.com.
166.com. 5 NS ns5.166.com.
166.com. 5 NS ns1.166.com.
166.com. 5 NS ns8.166.com.
166.com. 5 NS ns4.166.com.
Authority
The response has no authority records.
Additional
ns1.166.com. 5 A 42.186.168.126
ns2.166.com. 5 A 103.71.201.3
ns3.166.com. 5 A 182.92.22.240
ns4.166.com. 5 A 8.147.116.45
ns5.166.com. 5 A 103.71.201.3
ns6.166.com. 5 A 54.228.156.72
ns8.166.com. 5 A 18.182.82.158
ns8.166.com. 5 A 44.228.163.69
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0