A. The Datacenter edition of Windows Server is licensed on a per-processor. Each install must have at least two processors licensed, but that doesn't mean the actual hardware (physical or virtual) must have two processors. You can it install on a single processor system, you'll just need to pay for licenses for two processors.
One of the big advantages of Datacenter Edition when using virtualization is that you can run an unlimited number of virtual instances. If you're running a large Hyper-V server with Datacenter Edition, however, you'd probably have more than one physical processor anyway.