Which statement is correct about logical units on Juniper Router interfaces?（）

• A、Logical units are used only when a Layer 2 identifier is present such as a VLAN
• B、A logical unit of 0 is required when using a frame-relay DLCI
• C、A logical unit is always required
• D、Logical units are not required unless ATM or 802.1Q VLAN tagging is configured.