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？（）
- AThe stack bit will only be used when LDP is the label distribution protocol
- BThe label is the last entry in the label stack.
- CThe stack bit is for Cisco implementations exclusively and will only be used when TDP is the label distribution protocol.
- DThe stack bit is reserved for future use.