What does expandability mean for HP ZBook mobile workstations?

A.
upgradable capabilities with a slot for a CD/DVD

B.
upgradable memory and storage with easy access

C.
upgradable ports with security as a central focus

D.
upgradable screen resolution with removable monitor

Explanation:
http://h20195.www2.hp.com/V2/GetPDF.aspx/4AA4-8242ENW.pdf(page 6)