A major upgrade of the ESXi code on an existing vSAN cluster was just completed. Upon logging on to the vSphere Web Client, the vSphere administrator encounters UI timeouts on vSAN specific menus. What could cause this? 

Answer options:

A. vSphere Update Manager is offline.
B. vSAN Performance Service is offline.
C. vCenter Server has yet to be upgraded.
D. vSAN on-disk format has yet to be upgraded.

C