Which HP workstation boasts the most expandable HP chassis?

A.
HP Z820 workstation

B.
HP 2230 workstation

C.
HP Z420 workstation

D.
HP Z1 workstation

Explanation:
http://www8.hp.com/emea_africa/en/products/workstations/#!view=column&page=1(se
e HP Z820 workstation)