Apex domains cannot have CNAME records.
Question and response
Question
dig dc.xo.com. NS
Answer
dc.xo.com. 5 NS ns-tmtr.verizonwireless.com.
dc.xo.com. 5 NS ns-scrm.verizonwireless.com.
dc.xo.com. 5 NS ns-twbg.verizonwireless.com.
Authority
The response has no authority records.
Additional
ns-scrm.verizonwireless.com. 5 A 162.115.34.17
ns-tmtr.verizonwireless.com. 5 A 63.27.16.17
ns-twbg.verizonwireless.com. 5 A 137.188.98.17
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0