A group of nodes that represent a physical unit of failure. Defines a group of virtual machines that share a common set of hardware, switches, that share a single point of failure. For example, a server rack serviced by a set of power or networking switches. VMs in an availability set are placed in at least two ____________________s. This mitigates against the effects of hardware failures, network outages, power interruptions, or software updates. Think of a ____________________ as nodes belonging to the same physical rack.