(AC1987) Then, within the clause types, we further annotated the finite clauses -'that clause' (that-C) in (3.3a), 'wh-clause' (wh-C) in (3.3b), and whether/ifclause (whether/if-C) in (  Although core FEs are required at the semantic level by their frames, in some circumstances, they may be absent at the surface level.
they are not presented in the sentence, but exist at the semantic level.