What is true regarding the IEEE $802\mathrm{.}11$ WLAN standard?

a$.$ to avoid the crowded ISM spectrum, $802\mathrm{.}11$ requires the use of licensed frequency channels

b$.$ both centralized and decentralized access methods are available in the $802\mathrm{.}11$ standard

c$.$ WLAN communications can only be accomplished using an AP

d$.$ there is no method available to minimize data collisions over unguided medium