Apex domains cannot have CNAME records.
Question and response
Question
dig GCA.SU. NS
Answer
GCA.SU. 5 NS ns-mail02.getcourse.ru.
GCA.SU. 5 NS ns-mail01.getcourse.ru.
GCA.SU. 5 NS ns-mail03.getcourse.ru.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0