Which is the correct order for upgrading vSphere components?

Which is the correct order for upgrading vSphere components?
A . vCenter Server, ESXi hosts, VMware Tools, Virtual Machine hardware
B . VMware Tools, Virtual Machine hardware, ESXi hosts, vCenter Server
C . vCenter Server, Virtual Machine hardware, VMware tools, ESXi hosts
D . ESXi hosts, vCenter Server, Virtual Machine hardware, VMware Tools

Answer: A

Explanation: