Assuming TTL propagation is enabled, how does MPLS ensure the integrity of the TTL loop detection mechanism in IP?

A.
The TTL field in the MPLS header is set to 255. This field is decremented at each hop in the MPLS cloud. If the TTL has not expired, its value is copied back into the IP header.

B.
The TTL field in the IP header is copied to the TTL field in the MPLS header. This field is passed transparently through the MPLS cloud. The TTL value is copied back into the IP header.

C.
The TTL field in the IP header is copied to the TTL field in the MPLS header. This field is decremented by one in the MPLS cloud. If the TTL has not expired its value is copies back into the IP header.

D.
The TTL field in the IP header is copied to the TTL field in the MPLS header. This field is decremented at each hop in the MPLS cloud. If the TTL has not expired, its value is copied back into the IP header.