into spec-CP, regarding to ACP.
Pied-piping in prepositional structures
(32) (a) They asked [who he was referring to]
(b) They asked [to whom he was referring]
(a) informal style : leaving the preposition to
stranded or orphaned at the end
(b) formal style : the preposition to is pied-
piped along with the wh-pronoun whom