Multi Protocol Label Switching （MPLS） is a data-carrying mechanism that belongs to the family of packet-switched networks. For an MPLS label， if the stack bit is set to1， which option is true？（）

• A、The stack bit will only be used when LDP is the label distribution protocol
• B、The label is the last entry in the label stack.
• C、The stack bit is for Cisco implementations exclusively and will only be used when TDP is the label distribution protocol.
• D、The stack bit is reserved for future use.