What's the correct answer here?

Between 1993 and 2006, Mexico engaged in a major land certification program called Procede that gave property rights to some 2.5 million farm households.

The program was rolled out gradually across localities. Before certification, these farm households did not have ownership of the land and could be expropriated.

One research question is whether having a land title giving households security of land ownership creates opportunities for migration since they no longer risk losing the land. To test this, we can use data on migration from the population censuses by localities in 1980, 1990, and 2000.

We will measure the impact of the certification program by comparing the change in migration rates between 1990 and 2000 in treated localities (certified between 1993 and 1999) and not treated localities (certified in 2000 or after).

How would you verify that the difference-in-differences methodology would yield unbiased results?

Select one:

a.Compare the treatment and control group in terms of observable characteristics in 2000.

b.Test whether treatment and control groups were evolving along parallel trends before the treatment by comparing the change in migration rates between 1980 and 1990.

c.Test whether treatment and control groups were evolving along parallel trends before the treatment by comparing the change in migration rates between 1990 and 200.

d.Compare the treatment and control group in terms of observable characteristics in 1990.