When Might It Be the Case That a Difference-In-Differences Estimator

Multiple Choice

When might it be the case that a difference-in-differences estimator still does not identify a consistent estimate of the causal treatment effect?

A) If the treatment is non-normally distributed.
B) If there are non-time varying effects that generate differences between treated and non-treated groups.
C) If there are time varying effects that generate amongst both treated and non-treated groups symmetrically.
D) If there are time-varying trends that are different amongst treated and non-treated groups.