Apex domains cannot have CNAME records.
Question and response
Question
dig a1.b.yahoo.net. NS
Answer
a1.b.yahoo.net. 5 NS yf1.yahoo.com.
a1.b.yahoo.net. 5 NS yf2.yahoo.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0