(a) Except as provided by Subsection (b), a disability retirement annuity for service credited in the elected class of membership is an amount computed in the same manner as the standard service retirement annuity, not reduced because of age, for service credited in the elected class.
(b) An occupational disability retirement annuity for service credited in the elected class is computed on the basis of the amount of the member's service credit or eight years, whichever is greater.
(c) A person who retires under this section may select an optional retirement plan instead of the standard retirement annuity.