Apex domains cannot have CNAME records.
Question and response
Question
dig siteground.us. NS
Answer
siteground.us. 5 NS ns1.siteground.us.
siteground.us. 5 NS ns2.siteground.us.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0