Apex domains cannot have CNAME records.
Question and response
Question
dig in-addr.btopenworld.com. NS
Answer
in-addr.btopenworld.com. 5 NS a-ns.bt.com.
in-addr.btopenworld.com. 5 NS b-ns.bt.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0