A global customer’s IT strategy is based upon two DS8000 systems and an N series gateway. They want to protect their 
business by implementing a disaster recovery solution. The storage specialist has recommended cascaded mirroring. 
Which of the following is the business benefit of cascaded replication（）

A.it is cost efficient， as a single N series or DS8000 can have its controllers split between the  primary and secondary site and use semi- synchronousmirroring while using asynchronous mirroring to a third site
B.the second site acts as a repeater station so that greater geographical distances can be achieved
C.ITprovides high availability together with disaster recovery by implementing Metro/Global Mirror across three geographically dispersed systems
D.it guarantees data protection by implementing two synchronous copies of the primary site data