Apex domains cannot have CNAME records.
Question and response
Question
dig pbs.edu.pl. NS
Answer
pbs.edu.pl. 5 NS sun1.utp.edu.pl.
pbs.edu.pl. 5 NS dns.man.bydgoszcz.pl.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0