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.