What is the function of the command switchport trunk native vlan 999 on a Cisco Catalyst switch?

What is the function of the command switchport trunk native vlan 999 on a Cisco Catalyst switch?
A. It creates a VLAN 999 interface.
B. It designates VLAN 999 for untagged traffic.
C. It blocks VLAN 999 traffic from passing on the trunk.
D. It designates VLAN 999 as the default for all unknown tagged traffic.

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