Apex domains cannot have CNAME records.
Question and response
Question
dig cst.gov.sa. NS
Answer
cst.gov.sa. 5 NS ns1.cst.gov.sa.
cst.gov.sa. 5 NS ns2.cst.gov.sa.
cst.gov.sa. 5 NS ns3.cst.gov.sa.
Authority
The response has no authority records.
Additional
ns1.cst.gov.sa. 5 A 86.111.194.61
ns2.cst.gov.sa. 5 A 86.111.194.60
ns3.cst.gov.sa. 5 A 86.111.197.60
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0