Consider the panel regression model where t = 2. if some of the omitted variables, which you hope to capture in the changes analysis, in fact change over time, then the estimator on the included change regressor

(a) will be unbiased only when allowing for heteroskedastic-robust standard errors.

(b) may be biased.

(c) will only be unbiased in large samples.

(d) will always be unbiased.