What best describes server virtualization?

A.	Server virtualization is the masking of server resources (including the number and identity of individual physical servers, processors, and operating systems) from server users.
B.	System virtualization is the use of software to allow a piece of hardware to run multiple operating system images at the same time.
C.	A method of combining the available resources in a network by splitting up the available bandwidth into channels, each of which is independent from the others, and each of which can be assigned (or reassigned) to a particular server or device in real time.
D.	Pooling of physical storage from multiple network storage devices into what appears to be a single storage device that is managed from a central console.