(a) When a municipality that is part of an authority annexes territory that before the annexation is not part of the authority, the annexed territory becomes part of the authority.
(b) When a contiguous municipality annexes additional territory, the annexed territory becomes a part of the subregional authority created by the contiguous municipality.
(c) Except for Subsection (b), this subchapter does not apply to an authority created by a contiguous municipality.