Apex domains cannot have CNAME records.
Question and response
Question
dig sildegra100mg.com. NS
Answer
sildegra100mg.com. 5 NS ns-1920.awsdns-48.co.uk.
sildegra100mg.com. 5 NS ns-1720.awsdns-23.co.uk.
sildegra100mg.com. 5 NS ns-33.awsdns-04.com.
sildegra100mg.com. 5 NS ns-233.awsdns-29.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0