Which 4 statements regarding MPLS Label Stack Encoding is true?

Which 4 statements regarding MPLS Label Stack Encoding is true?

A.
A value of 4 represents the "Implicit NULL Label."

B.
A value of 0 represents the "IPv4 Explicit NULL Label."

C.
A value of 1 represents the "Router Alert Label". The use of this label is analogous to the use of the "Router Alert Option" in IP packets (for example, ping with record route option).

D.
A value of 2 represents the "IPv6 Explicit NULL Label"

E.
A value of 1 represents the "IPv1 Explicit NULL Label"

F.
A value of 3 represents the "Implicit NULL Label"