Apex domains cannot have CNAME records.
Question and response
Question
dig gdc.gov.sg. NS
Answer
gdc.gov.sg. 3 NS ns7.gdnsdef.com.
gdc.gov.sg. 3 NS a3-67.akam.net.
gdc.gov.sg. 3 NS a20-66.akam.net.
gdc.gov.sg. 3 NS a2-65.akam.net.
gdc.gov.sg. 3 NS ns12.gdnsec.com.
gdc.gov.sg. 3 NS a1-6.akam.net.
gdc.gov.sg. 3 NS ns10.gdnsdef.com.
gdc.gov.sg. 3 NS ns9.gdnsec.com.
gdc.gov.sg. 3 NS a8-64.akam.net.
gdc.gov.sg. 3 NS a9-65.akam.net.
gdc.gov.sg. 3 NS ns12.gdnsdef.com.
gdc.gov.sg. 3 NS ns3.gdnsec.com.
Authority
The response has no authority records.
Additional
ns3.gdnsec.com. 1 A 45.116.40.3
ns7.gdnsdef.com. 1 A 45.116.42.7
ns9.gdnsec.com. 1 A 45.116.40.9
a1-6.akam.net. 1 A 193.108.91.6
ns10.gdnsdef.com. 1 A 45.116.42.10
ns12.gdnsec.com. 1 A 45.116.40.12
ns12.gdnsdef.com. 1 A 45.116.42.12
a2-65.akam.net. 1 A 95.100.174.65
a3-67.akam.net. 1 A 96.7.49.67
a8-64.akam.net. 1 A 2.16.40.64
a9-65.akam.net. 1 A 184.85.248.65
a20-66.akam.net. 1 A 95.100.175.66
ns3.gdnsec.com. 1 AAAA 2404:ae00:3266:0:0:0:2d74:2802
ns7.gdnsdef.com. 1 AAAA 2404:ae00:3266:0:0:0:2d74:2a06
ns9.gdnsec.com. 1 AAAA 2404:ae00:3266:0:0:0:2d74:2808
a1-6.akam.net. 1 AAAA 2600:1401:2:0:0:0:0:6
ns10.gdnsdef.com. 1 AAAA 2404:ae00:3266:0:0:0:2d74:2a09
ns12.gdnsec.com. 1 AAAA 2404:ae00:3266:0:0:0:2d74:280b
ns12.gdnsdef.com. 1 AAAA 2404:ae00:3266:0:0:0:2d74:2a0b
a2-65.akam.net. 1 AAAA 2600:1480:7000:0:0:0:0:41
a3-67.akam.net. 1 AAAA 2600:1408:1c:0:0:0:0:43
a8-64.akam.net. 1 AAAA 2600:1403:a:0:0:0:0:40
a9-65.akam.net. 1 AAAA 2a02:26f0:117:0:0:0:0:41
a20-66.akam.net. 1 AAAA 2a02:26f0:67:0:0:0:0:42
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0