Apex domains cannot have CNAME records.
Question and response
Question
dig onlinebridgetutor.com. NS
Answer
onlinebridgetutor.com. 1 NS ns1.onlinebridgetutor.com.
onlinebridgetutor.com. 1 NS ns2.onlinebridgetutor.com.
Authority
The response has no authority records.
Additional
ns1.onlinebridgetutor.com. 1 A 50.2.108.6
ns2.onlinebridgetutor.com. 1 A 50.2.115.173
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0