Juan has just developed the ability to convert a declarative sentence $($such as$,$I like pizza$)$ into both a question $($such as$,$Do I like pizza?$)$ and a negative sentence $($such as $$I don$$t like pizza$).$ Juan$$s use of these sentences illustrates

a$.$ functional grammar.

b$.$ overextension.

c$.$ transformational grammar.

d$.$ overregularization.