a) A virtual machine is a computer that is walled off from the physical computer that the virtual machine is running on
b) Virtual machines provide the capability of running multiple machine instances, each with their own operating system
c) The downside of virtual machine technologies is that having resources indirectly addressed means there is some level of overhead
d) All of the mentioned

Answer: d
Explanation: This makes virtual machine technology very useful for running old versions of operating systems, testing applications in what amounts to a sandbox.