S$->$AB$,$ A$->$Ca$,$ B$->$bD$,$ C$->$XY$,$ D$->$C$,$ X$->$aX$|$bX$|$null$,$ Y$->$Ya$|$Yb$|$null

$($Choose all that apply!$)$

Group of answer choices

only S

None is nullable non$-$terminal

only C and D

C$,$ D$,$ X$,$ Y