Apex domains cannot have CNAME records.
Question and response
Question
dig pearsontestprep.com. NS
Answer
pearsontestprep.com. 4 NS dns-apac-1.ptns.pearson.com.
pearsontestprep.com. 4 NS dns-amer-2.ptns.pearson.com.
pearsontestprep.com. 4 NS dns-emea-1.ptns.pearson.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0