Sun Stable is a global insurance company with headquarters located in Houston, Texas. The campus there is made up of a number of office buildings located within the same vicinity. In 2003, a new building, Building 331B was added. The additional building houses approximately 1000 employees. Rather than deploy a private branch exchange (PBX) in the new building, Sun Stable has decided to implement an IP telephony solution. External calls will be carried across a MAN link to another building, where a gateway connects into the worldwide PBX network of Sun Stable. Voice mail and unified messaging components are required and all IP phones and workstations should be on separate VLANs and IP subnets.Which IP telephony deployment best suits their need?（）

A. single-site.
B. multisite with centralized call processing
C. multisite with distributed call processing
D. clustering over the WAN