Which one of these statements is true about addressing redundancy within the WAN environment?

Which one of these statements is true about addressing redundancy within the WAN environment?

A.
The reliability and speed of DSL allow for cost savings by not including redundant links.

B.
CAMDM and dark fiber offer advanced redundancy features such as automatic backup and repair mechanism to cope system faults.

C.
An SLA is one way to eliminate the need for redundancy.

D.
The failure of a single SONET/SDH link or network element does not lead to failure of the entire network.

Explanation:
Dual-Ring topologies are being used in WAN environment.
Link: http://en.wikipedia.org/wiki/Ring_network