Select the correct statement$($s$)$ regarding IEEE $802\mathrm{.}11$ WLAN.

a$.$ to minimize RFI $($RF interference$),$ WLANs implement either DSSS $($direct sequence spread spectrum$)$ or OFDM $($orthogonal frequency division multiplexing$)$

b$.$ increased data capacity over a WLAN can be achieved by implementing M'ary modulation schemes

c$.$ MIMO $($multiple in$,$ multiple out$)$ antenna arrays are used in some $802\mathrm{.}11$ specifications

d$.$ all statements correct