Models of Language Often Look Like a 'Double Hierarchy,' Because

Multiple Choice

Models of language often look like a 'double hierarchy,' because

A) there are structural hierarchies in both the flow of input and output.
B) ambiguous sentences, such as "They are flying airplanes", have double interpretations, resulting in one hierarchy for each interpretation.
C) there are both top-down and bottom-up hierarchies in language processing.
D) all of the above