Suppose you want to estimate the effect of a recently introduced transfer program on female labor supply. You have data on labor supply before and after the program was introduced. Assume that women with children are eligible for the program and women without children are ineligible. (a) Write down a difference-in-difference estimator you could use to estimate the effect of the program. Make sure your variables are defined clearly enough that someone with basic statistics knowledge could implement this in the data. (b) What is the identifying assumption, in words? Please be specific to this particular case.