About the description of MUX VLAN, which is wrong?

About the description of MUX VLAN, which is wrong?
A . Each Group VLAN must have a Principal VLAN bound to it.
B . If the VLAN ID is already used for the principal VLAN, the VLAN can no longer be used for VLANIF interfaces, super VLANs and sub VLANs.
C . After MUX VLAN is enabled on a port, you can use it again for VLAN mapping and VLAN stacking configuration.
D . Enterprises can use the Principal PORT to connect to enterprise servers, separate PORT to connect enterprise customers and Group PORT to connect enterprise employees. In this way, enterprise customers and enterprise employees can access enterprise servers, and internal employees can communicate, enterprise customers cannot communicate, and enterprise customers and enterprise employees cannot communicate with each other.

Answer: C