Again, start with the original model, but now allow wages to differ across four groups of people: a. married and black b. married and nonblack c. single and black, and single and nonblack. what is the estimated wage differential between married blacks and married nonblacks?

"six months ago, several of molly's friends joined the trimtime fitness center. each of them participated in trimtime's weight-reduction and fitness regimen. all reported substantial weight reduction, and all are visibly slimmer. molly is convinced. she joins trimtime and enrolls in the program, hoping and expecting to see the same results. she is especially delighted to learn that trimtime had adjusted its program to make it even more effective in a shorter period of time." what is the feature in question/attribute of interest in this argument?