While calculating a difference in difference, we run a regression which is as follows: lm( y ~ d1 +d2 + d3) where d1 and d2 are dummy variables and d3 is their interaction term. We thus get its coefficients according to the equation: Y = a + b*d1 + c*d2 + d*d3

What is the difference in difference estimator?
a. a+b+c+d
b. (d-c)-(b-a)
c. a
d. b