Explanation/Reference:
A: Computer hardware virtualization (or hardware virtualisation) is the virtualization of computers or operating systems. It hides the physical characteristics of a computing platform from users, instead showing another abstract computing platform.
C: Computer clusters have historically run on separate physical computers with the same operating system. With the advent of virtualization, the cluster nodes may run on separate physical computers with different operating systems which are painted above with a virtual layer to look similar. The cluster may also be virtualized on various configurations as maintenance takes place.
Incorrect answers:
B: Virtualization is a software concept.
D: Virtualization is a key technology used in datacenters to optimize resources.
Reference: Oracle White Paper, The Most Complete and Integrated Virtualization: From Desktop to Datacenter