$15.$ You are asked to design an $802\mathrm{.}1$X$/$EAP solution for the corporate WLAN. The company headquarters and datacenter reside in Denver. Employees need secure WLAN access at $15$ remote offices in other cities. Which of these RADIUS deployment models guarantees secure WLAN connectivity even if a remote WAN link goes down?

A$.$ Single$-$site deployment

B$.$ Distributed autonomous sites

C$.$ Distributed sites, centralized RADIUS and LDAP

D$.$ Distributed sites with RADIUS, centralized LDAP