在vCenter服务器升级，一个在集群高可用性群组的ESXi 6.x主机发生故障。哪个说法是正确的？（）

A.HA will successfully vMotion the virtual machines during the host failure.
B.HA will fail the virtual machines over to an available host after the vCenter Server upgrade completes.
C.HA will fail the virtual machines over to an available host during the vCenter Server upgrade process.
D.HA is unavailable during the vCenter Server upgrade process.