In the Context of Modeling the Impact of Unionization, the Simultaneity

In the context of modeling the impact of unionization, the simultaneity issue refers to the fact that:

A) Longitudinal data is used, which renders the estimation of the union wage effect.
B) The union status of a worker and the union wage effect are jointly determined, which renders the estimation of the union wage effect.
C) Union wages are determined at the same time as non-union wages, which renders the estimation of the union wage effect.
D) The threat effect can operate at the same times as the wait-unemployment effect, which renders the estimation of the union wage effect.
E) There is variation in the union wage impact across types of firms and workers, which renders the estimation of the union wage effect.